The C# PDF Library

HTML to PDF

using IronPdf;

// Disable local disk access or cross-origin requests
Installation.EnableWebSecurity = true;

// Instantiate Renderer
var renderer = new ChromePdfRenderer();

// Create a PDF from a HTML string using C#
var pdf = renderer.RenderHtmlAsPdf("<h1>Hello World</h1>");

// Export to a file or Stream
pdf.SaveAs("output.pdf");

// Advanced Example with HTML Assets
// Load external html assets: Images, CSS and JavaScript.
// An optional BasePath 'C:\site\assets\' is set as the file location to load assets from
var myAdvancedPdf = renderer.RenderHtmlAsPdf("<img src='icons/iron.png'>", @"C:\site\assets\");
myAdvancedPdf.SaveAs("html-with-assets.pdf");

using IronPdf;

// Disable local disk access or cross-origin requests
Installation.EnableWebSecurity = true;

// Instantiate Renderer
var renderer = new ChromePdfRenderer();

// Create a PDF from a HTML string using C#
var pdf = renderer.RenderHtmlAsPdf("<h1>Hello World</h1>");

// Export to a file or Stream
pdf.SaveAs("output.pdf");

// Advanced Example with HTML Assets
// Load external html assets: Images, CSS and JavaScript.
// An optional BasePath 'C:\site\assets\' is set as the file location to load assets from
var myAdvancedPdf = renderer.RenderHtmlAsPdf("<img src='icons/iron.png'>", @"C:\site\assets\");
myAdvancedPdf.SaveAs("html-with-assets.pdf");

PM >Install-Package IronPdf

Install-Package IronPdf

MIGRATION GUIDES

How to Migrate from Telerik Document Processing to IronPDF in C#

Curtis Chau

Updated: August 1, 2026

Telerik Document Processing ships as part of the Telerik UI suites and DevCraft bundles, offering developers PDF generation capabilities alongside Word, Excel, and PowerPoint processing. However, as modern web standards evolve and projects demand full CSS3, Flexbox, and Bootstrap support, development teams have voiced concerns about limitations in Telerik's HTML-to-PDF rendering architecture.

This guide provides a complete migration path from Telerik Document Processing to IronPDF, with step-by-step instructions, code comparisons, and practical examples drawn directly from real migration scenarios.

Why Migrate from Telerik Document Processing

The decision to migrate from Telerik Document Processing typically stems from technical limitations that become apparent when working with modern web content. Understanding these limitations helps justify the migration effort and sets expectations for what IronPDF resolves.

Reported Technical Limitations

Developers have reported issues with Telerik Document Processing when handling modern HTML and CSS:

Issue	Impact	IronPDF Solution
CSS parsing limitations	Modern CSS frameworks like Bootstrap fail	Full Chromium CSS support
Div-to-paragraph conversion	HTML structure flattened, layouts break	Direct HTML rendering
Flow document model	Forces intermediate conversion	Native HTML-to-PDF
External CSS issues	Complex selectors ignored	Full CSS file support
Memory issues	`OutOfMemoryException` reported on large files	Efficient streaming

The Core Problem: HTML Is Not Rendered Correctly

Telerik Document Processing converts HTML to an intermediate "Flow Document" model before generating PDF. This architectural decision creates cascading problems:

Flattens HTML structure - <div> elements become paragraphs
Ignores modern CSS - Flexbox and Grid layouts fail completely
Breaks Bootstrap - Column systems don't render as columns
Loses formatting - Complex CSS selectors are ignored

Consider this modern HTML that uses standard CSS layouts:

<!-- This modern HTML/CSS BREAKS in Telerik Document Processing -->
<div class="container">
    <div class="row">
        <div class="col-md-6">Column 1</div>
        <div class="col-md-6">Column 2</div>
    </div>
</div>

<div style="display: flex; gap: 20px;">
    <div style="flex: 1;">Flex Item 1</div>
    <div style="flex: 1;">Flex Item 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr);">
    <div>Grid Item 1</div>
    <div>Grid Item 2</div>
    <div>Grid Item 3</div>
</div>

<!-- This modern HTML/CSS BREAKS in Telerik Document Processing -->
<div class="container">
    <div class="row">
        <div class="col-md-6">Column 1</div>
        <div class="col-md-6">Column 2</div>
    </div>
</div>

<div style="display: flex; gap: 20px;">
    <div style="flex: 1;">Flex Item 1</div>
    <div style="flex: 1;">Flex Item 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr);">
    <div>Grid Item 1</div>
    <div>Grid Item 2</div>
    <div>Grid Item 3</div>
</div>

HTML

In Telerik Document Processing, all of these layouts render as sequential paragraphs. The two-column Bootstrap layout becomes two lines of text. The flexbox items stack vertically. The CSS Grid items appear one after another.

CSS Features That Fail in Telerik Document Processing

Developers have documented extensive CSS compatibility issues:

/* ❌ These CSS features DON'T WORK in Telerik Document Processing */

/* Flexbox - Not supported */
.container { display: flex; }
.item { flex: 1; }

/* CSS Grid - Not supported */
.grid { display: grid; grid-template-columns: repeat(3, 1fr); }

/* Bootstrap columns - Converted to paragraphs */
.col-md-6 { /* Ignored, becomes linear text */ }

/* CSS Variables - Not supported */
:root { --primary: #007bff; }
.btn { color: var(--primary); }

/* Complex selectors - Often ignored */
.container > .row:first-child { }
.item:hover { }
.content::before { }

/* Modern units - Limited support */
.box { width: calc(100% - 20px); }
.text { font-size: 1.2rem; }

/* ❌ These CSS features DON'T WORK in Telerik Document Processing */

/* Flexbox - Not supported */
.container { display: flex; }
.item { flex: 1; }

/* CSS Grid - Not supported */
.grid { display: grid; grid-template-columns: repeat(3, 1fr); }

/* Bootstrap columns - Converted to paragraphs */
.col-md-6 { /* Ignored, becomes linear text */ }

/* CSS Variables - Not supported */
:root { --primary: #007bff; }
.btn { color: var(--primary); }

/* Complex selectors - Often ignored */
.container > .row:first-child { }
.item:hover { }
.content::before { }

/* Modern units - Limited support */
.box { width: calc(100% - 20px); }
.text { font-size: 1.2rem; }

Text

Performance Reports for Large Documents

Beyond CSS limitations, developers have reported OutOfMemoryException errors when processing large files with Telerik Document Processing, along with unsupported color spaces and crashes during PDF merging in some scenarios. Teams working with high-volume document generation have noted stability concerns worth evaluating before committing to long-term use.

IronPDF vs Telerik Document Processing: Feature Comparison

Understanding the feature differences helps technical decision-makers evaluate the migration investment:

Feature	Telerik Document Processing	IronPDF
HTML Rendering	Flow Document conversion	Direct Chromium rendering
CSS3 Support	Limited, many features fail	Full CSS3
Flexbox	Not supported	Supported
CSS Grid	Not supported	Supported
Bootstrap	Broken (div flattening)	Supported including Bootstrap 5
External CSS	Partial	Supported
JavaScript	Not supported	Full support
Large Documents	Memory issues reported	Efficient streaming
API Complexity	Complex (providers, models)	Simple (one class)
License Model	Commercial, part of DevCraft	Simple standalone licensing

Quick Start Migration: 5 Minutes to Your First PDF

The migration from Telerik Document Processing to IronPDF can begin immediately. Here's the fastest path to generating your first PDF with IronPDF.

Step 1: Update NuGet Packages

Remove all Telerik document processing packages:

# Remove Telerik packages
dotnet remove package Telerik.Documents.Core
dotnet remove package Telerik.Documents.Flow
dotnet remove package Telerik.Documents.Flow.FormatProviders.Pdf
dotnet remove package Telerik.Documents.Fixed

# Remove Telerik packages
dotnet remove package Telerik.Documents.Core
dotnet remove package Telerik.Documents.Flow
dotnet remove package Telerik.Documents.Flow.FormatProviders.Pdf
dotnet remove package Telerik.Documents.Fixed

SHELL

Install IronPDF:

# Install IronPDF
dotnet add package IronPdf

# Install IronPDF
dotnet add package IronPdf

SHELL

Step 2: Update Using Statements

Replace Telerik namespaces with the IronPDF namespace:

// Before (Telerik Document Processing)
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using Telerik.Documents.Primitives;

// After (IronPDF)
using IronPdf;

// Before (Telerik Document Processing)
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using Telerik.Documents.Primitives;

// After (IronPDF)
using IronPdf;

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports Telerik.Documents.Primitives

' After (IronPDF)
Imports IronPdf

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports Telerik.Documents.Primitives

' After (IronPDF)
Imports IronPdf

Step 3: Add License Key

Initialize your license at application startup:

// Add at application startup
IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY";

// Add at application startup
IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY";

' Add at application startup
IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY"

' Add at application startup
IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY"

Step 4: Update Your PDF Generation Code

The transformation from Telerik Document Processing to IronPDF dramatically simplifies your codebase.

Before (Telerik Document Processing):

using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(htmlContent);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
byte[] pdfBytes = pdfProvider.Export(document);

File.WriteAllBytes("output.pdf", pdfBytes);

using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(htmlContent);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
byte[] pdfBytes = pdfProvider.Export(document);

File.WriteAllBytes("output.pdf", pdfBytes);

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model

Dim htmlProvider As New HtmlFormatProvider()
Dim document As RadFlowDocument = htmlProvider.Import(htmlContent)

Dim pdfProvider As New PdfFormatProvider()
Dim pdfBytes As Byte() = pdfProvider.Export(document)

File.WriteAllBytes("output.pdf", pdfBytes)

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model

Dim htmlProvider As New HtmlFormatProvider()
Dim document As RadFlowDocument = htmlProvider.Import(htmlContent)

Dim pdfProvider As New PdfFormatProvider()
Dim pdfBytes As Byte() = pdfProvider.Export(document)

File.WriteAllBytes("output.pdf", pdfBytes)

After (IronPDF):

using IronPdf;

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf(htmlContent);
pdf.SaveAs("output.pdf");

using IronPdf;

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf(htmlContent);
pdf.SaveAs("output.pdf");

Imports IronPdf

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderHtmlAsPdf(htmlContent)
pdf.SaveAs("output.pdf")

Imports IronPdf

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderHtmlAsPdf(htmlContent)
pdf.SaveAs("output.pdf")

The difference is immediately apparent: 3 lines of IronPDF code replace 15+ lines of Telerik Document Processing code. More importantly, the IronPDF version will correctly render modern CSS that Telerik's Flow Document model cannot handle.

Code Migration Examples

Converting HTML to PDF

This is the most common use case for PDF generation. The architectural differences between Telerik Document Processing and IronPDF become clear in this comparison.

Telerik Document Processing Approach:

// NuGet: Install-Package Telerik.Documents.Flow
// NuGet: Install-Package Telerik.Documents.Flow.FormatProviders.Pdf
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using System.IO;

string html = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>";

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(html);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
using (FileStream output = File.OpenWrite("output.pdf"))
{
    pdfProvider.Export(document, output);
}

// NuGet: Install-Package Telerik.Documents.Flow
// NuGet: Install-Package Telerik.Documents.Flow.FormatProviders.Pdf
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using System.IO;

string html = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>";

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(html);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
using (FileStream output = File.OpenWrite("output.pdf"))
{
    pdfProvider.Export(document, output);
}

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports System.IO

Dim html As String = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>"

Dim htmlProvider As New HtmlFormatProvider()
Dim document As RadFlowDocument = htmlProvider.Import(html)

Dim pdfProvider As New PdfFormatProvider()
Using output As FileStream = File.OpenWrite("output.pdf")
    pdfProvider.Export(document, output)
End Using

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports System.IO

Dim html As String = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>"

Dim htmlProvider As New HtmlFormatProvider()
Dim document As RadFlowDocument = htmlProvider.Import(html)

Dim pdfProvider As New PdfFormatProvider()
Using output As FileStream = File.OpenWrite("output.pdf")
    pdfProvider.Export(document, output)
End Using

IronPDF Approach:

// NuGet: Install-Package IronPdf
using IronPdf;

string html = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>";

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf(html);
pdf.SaveAs("output.pdf");

// NuGet: Install-Package IronPdf
using IronPdf;

string html = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>";

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf(html);
pdf.SaveAs("output.pdf");

Imports IronPdf

Dim html As String = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>"

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderHtmlAsPdf(html)
pdf.SaveAs("output.pdf")

Imports IronPdf

Dim html As String = "<html><body><h1>Hello World</h1><p>This is a PDF document.</p></body></html>"

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderHtmlAsPdf(html)
pdf.SaveAs("output.pdf")

The Telerik version requires creating an HtmlFormatProvider, importing to a RadFlowDocument, creating a PdfFormatProvider, and managing file streams manually. IronPDF's ChromePdfRenderer handles the entire process with a single method call.

For more advanced HTML-to-PDF scenarios, see the HTML to PDF conversion guide.

Converting URLs to PDF

Capturing web pages as PDFs reveals another significant difference between the libraries.

Telerik Document Processing Approach:

// NuGet: Install-Package Telerik.Documents.Flow
// NuGet: Install-Package Telerik.Documents.Flow.FormatProviders.Pdf
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

string url = "https://example.com";

using HttpClient client = new HttpClient();
string html = await client.GetStringAsync(url);

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(html);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
using (FileStream output = File.OpenWrite("webpage.pdf"))
{
    pdfProvider.Export(document, output);
}

// NuGet: Install-Package Telerik.Documents.Flow
// NuGet: Install-Package Telerik.Documents.Flow.FormatProviders.Pdf
using Telerik.Windows.Documents.Flow.FormatProviders.Html;
using Telerik.Windows.Documents.Flow.FormatProviders.Pdf;
using Telerik.Windows.Documents.Flow.Model;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

string url = "https://example.com";

using HttpClient client = new HttpClient();
string html = await client.GetStringAsync(url);

HtmlFormatProvider htmlProvider = new HtmlFormatProvider();
RadFlowDocument document = htmlProvider.Import(html);

PdfFormatProvider pdfProvider = new PdfFormatProvider();
using (FileStream output = File.OpenWrite("webpage.pdf"))
{
    pdfProvider.Export(document, output);
}

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports System.IO
Imports System.Net.Http
Imports System.Threading.Tasks

Module Module1
    Sub Main()
        Dim url As String = "https://example.com"

        Using client As New HttpClient()
            Dim html As String = Await client.GetStringAsync(url)

            Dim htmlProvider As New HtmlFormatProvider()
            Dim document As RadFlowDocument = htmlProvider.Import(html)

            Dim pdfProvider As New PdfFormatProvider()
            Using output As FileStream = File.OpenWrite("webpage.pdf")
                pdfProvider.Export(document, output)
            End Using
        End Using
    End Sub
End Module

Imports Telerik.Windows.Documents.Flow.FormatProviders.Html
Imports Telerik.Windows.Documents.Flow.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Flow.Model
Imports System.IO
Imports System.Net.Http
Imports System.Threading.Tasks

Module Module1
    Sub Main()
        Dim url As String = "https://example.com"

        Using client As New HttpClient()
            Dim html As String = Await client.GetStringAsync(url)

            Dim htmlProvider As New HtmlFormatProvider()
            Dim document As RadFlowDocument = htmlProvider.Import(html)

            Dim pdfProvider As New PdfFormatProvider()
            Using output As FileStream = File.OpenWrite("webpage.pdf")
                pdfProvider.Export(document, output)
            End Using
        End Using
    End Sub
End Module

IronPDF Approach:

// NuGet: Install-Package IronPdf
using IronPdf;

string url = "https://example.com";

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderUrlAsPdf(url);
pdf.SaveAs("webpage.pdf");

// NuGet: Install-Package IronPdf
using IronPdf;

string url = "https://example.com";

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderUrlAsPdf(url);
pdf.SaveAs("webpage.pdf");

Imports IronPdf

Dim url As String = "https://example.com"

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderUrlAsPdf(url)
pdf.SaveAs("webpage.pdf")

Imports IronPdf

Dim url As String = "https://example.com"

Dim renderer As New ChromePdfRenderer()
Dim pdf = renderer.RenderUrlAsPdf(url)
pdf.SaveAs("webpage.pdf")

Telerik Document Processing has no native URL-to-PDF capability. You must manually fetch HTML content using HttpClient, losing external CSS, JavaScript execution, and dynamic content in the process. IronPDF's RenderUrlAsPdf method captures the complete rendered page exactly as it appears in a browser.

Explore the URL to PDF documentation for additional options including authentication and custom headers.

Merging Multiple PDFs

PDF merging demonstrates the verbosity difference between these .NET PDF libraries.

Telerik Document Processing Approach:

// NuGet: Install-Package Telerik.Documents.Fixed
using Telerik.Windows.Documents.Fixed.FormatProviders.Pdf;
using Telerik.Windows.Documents.Fixed.Model;
using System.IO;

PdfFormatProvider provider = new PdfFormatProvider();

RadFixedDocument document1;
using (FileStream input = File.OpenRead("document1.pdf"))
{
    document1 = provider.Import(input);
}

RadFixedDocument document2;
using (FileStream input = File.OpenRead("document2.pdf"))
{
    document2 = provider.Import(input);
}

RadFixedDocument mergedDocument = new RadFixedDocument();
foreach (var page in document1.Pages)
{
    mergedDocument.Pages.Add(page);
}
foreach (var page in document2.Pages)
{
    mergedDocument.Pages.Add(page);
}

using (FileStream output = File.OpenWrite("merged.pdf"))
{
    provider.Export(mergedDocument, output);
}

// NuGet: Install-Package Telerik.Documents.Fixed
using Telerik.Windows.Documents.Fixed.FormatProviders.Pdf;
using Telerik.Windows.Documents.Fixed.Model;
using System.IO;

PdfFormatProvider provider = new PdfFormatProvider();

RadFixedDocument document1;
using (FileStream input = File.OpenRead("document1.pdf"))
{
    document1 = provider.Import(input);
}

RadFixedDocument document2;
using (FileStream input = File.OpenRead("document2.pdf"))
{
    document2 = provider.Import(input);
}

RadFixedDocument mergedDocument = new RadFixedDocument();
foreach (var page in document1.Pages)
{
    mergedDocument.Pages.Add(page);
}
foreach (var page in document2.Pages)
{
    mergedDocument.Pages.Add(page);
}

using (FileStream output = File.OpenWrite("merged.pdf"))
{
    provider.Export(mergedDocument, output);
}

Imports Telerik.Windows.Documents.Fixed.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Fixed.Model
Imports System.IO

Dim provider As New PdfFormatProvider()

Dim document1 As RadFixedDocument
Using input As FileStream = File.OpenRead("document1.pdf")
    document1 = provider.Import(input)
End Using

Dim document2 As RadFixedDocument
Using input As FileStream = File.OpenRead("document2.pdf")
    document2 = provider.Import(input)
End Using

Dim mergedDocument As New RadFixedDocument()
For Each page In document1.Pages
    mergedDocument.Pages.Add(page)
Next
For Each page In document2.Pages
    mergedDocument.Pages.Add(page)
Next

Using output As FileStream = File.OpenWrite("merged.pdf")
    provider.Export(mergedDocument, output)
End Using

Imports Telerik.Windows.Documents.Fixed.FormatProviders.Pdf
Imports Telerik.Windows.Documents.Fixed.Model
Imports System.IO

Dim provider As New PdfFormatProvider()

Dim document1 As RadFixedDocument
Using input As FileStream = File.OpenRead("document1.pdf")
    document1 = provider.Import(input)
End Using

Dim document2 As RadFixedDocument
Using input As FileStream = File.OpenRead("document2.pdf")
    document2 = provider.Import(input)
End Using

Dim mergedDocument As New RadFixedDocument()
For Each page In document1.Pages
    mergedDocument.Pages.Add(page)
Next
For Each page In document2.Pages
    mergedDocument.Pages.Add(page)
Next

Using output As FileStream = File.OpenWrite("merged.pdf")
    provider.Export(mergedDocument, output)
End Using

IronPDF Approach:

// NuGet: Install-Package IronPdf
using IronPdf;

var pdf1 = PdfDocument.FromFile("document1.pdf");
var pdf2 = PdfDocument.FromFile("document2.pdf");

var merged = PdfDocument.Merge(pdf1, pdf2);
merged.SaveAs("merged.pdf");

// NuGet: Install-Package IronPdf
using IronPdf;

var pdf1 = PdfDocument.FromFile("document1.pdf");
var pdf2 = PdfDocument.FromFile("document2.pdf");

var merged = PdfDocument.Merge(pdf1, pdf2);
merged.SaveAs("merged.pdf");

Imports IronPdf

Dim pdf1 = PdfDocument.FromFile("document1.pdf")
Dim pdf2 = PdfDocument.FromFile("document2.pdf")

Dim merged = PdfDocument.Merge(pdf1, pdf2)
merged.SaveAs("merged.pdf")

Imports IronPdf

Dim pdf1 = PdfDocument.FromFile("document1.pdf")
Dim pdf2 = PdfDocument.FromFile("document2.pdf")

Dim merged = PdfDocument.Merge(pdf1, pdf2)
merged.SaveAs("merged.pdf")

The Telerik version requires importing each document separately, manually iterating through pages, adding them to a new document, and managing multiple file streams. IronPDF's PdfDocument.Merge() method handles everything in a single call.

For advanced merging scenarios including selective page extraction, see the merge and split PDFs guide.

Telerik Document Processing API to IronPDF Mapping

This reference table accelerates migration by showing direct API equivalents:

Telerik Document Processing	IronPDF
`HtmlFormatProvider`	`ChromePdfRenderer`
`RadFlowDocument`	Not needed
`PdfFormatProvider`	`pdf.SaveAs()`
`RadFlowDocumentEditor`	HTML manipulation
`Section`	HTML `<section>`
`Paragraph`	HTML `<p>`
`PdfExportSettings`	`RenderingOptions`
`RadFixedDocument`	`PdfDocument`
Manual page iteration	`PdfDocument.Merge()`

Handling Headers and Footers

Telerik Document Processing uses a programmatic model for headers and footers. IronPDF provides HTML-based headers with dynamic placeholders.

Telerik Document Processing:

section.Headers.Default.Blocks.AddParagraph().Inlines.AddRun("Header Text");
section.Footers.Default.Blocks.AddParagraph().Inlines.AddRun("Footer Text");

section.Headers.Default.Blocks.AddParagraph().Inlines.AddRun("Header Text");
section.Footers.Default.Blocks.AddParagraph().Inlines.AddRun("Footer Text");

section.Headers.Default.Blocks.AddParagraph().Inlines.AddRun("Header Text")
section.Footers.Default.Blocks.AddParagraph().Inlines.AddRun("Footer Text")

section.Headers.Default.Blocks.AddParagraph().Inlines.AddRun("Header Text")
section.Footers.Default.Blocks.AddParagraph().Inlines.AddRun("Footer Text")

IronPDF:

renderer.RenderingOptions.HtmlHeader = new HtmlHeaderFooter()
{
    HtmlFragment = "<div style='text-align:center;'>Header Text</div>",
    MaxHeight = 25
};
renderer.RenderingOptions.HtmlFooter = new HtmlHeaderFooter()
{
    HtmlFragment = "<div style='text-align:center;'>Footer Text</div>",
    MaxHeight = 25
};

renderer.RenderingOptions.HtmlHeader = new HtmlHeaderFooter()
{
    HtmlFragment = "<div style='text-align:center;'>Header Text</div>",
    MaxHeight = 25
};
renderer.RenderingOptions.HtmlFooter = new HtmlHeaderFooter()
{
    HtmlFragment = "<div style='text-align:center;'>Footer Text</div>",
    MaxHeight = 25
};

Imports System

renderer.RenderingOptions.HtmlHeader = New HtmlHeaderFooter() With {
    .HtmlFragment = "<div style='text-align:center;'>Header Text</div>",
    .MaxHeight = 25
}
renderer.RenderingOptions.HtmlFooter = New HtmlHeaderFooter() With {
    .HtmlFragment = "<div style='text-align:center;'>Footer Text</div>",
    .MaxHeight = 25
}

Imports System

renderer.RenderingOptions.HtmlHeader = New HtmlHeaderFooter() With {
    .HtmlFragment = "<div style='text-align:center;'>Header Text</div>",
    .MaxHeight = 25
}
renderer.RenderingOptions.HtmlFooter = New HtmlHeaderFooter() With {
    .HtmlFragment = "<div style='text-align:center;'>Footer Text</div>",
    .MaxHeight = 25
}

IronPDF's HTML-based approach allows full CSS styling in headers and footers, plus dynamic placeholders for page numbers and dates. Learn more in the headers and footers documentation.

Unit Conversion: DIPs to Millimeters

Telerik Document Processing uses device-independent pixels (DIPs) for measurements. IronPDF uses millimeters. This requires conversion during migration:

// Telerik uses DIPs (device-independent pixels)
// IronPDF uses millimeters

// 1 inch = 25.4mm
renderer.RenderingOptions.MarginTop = 25.4;    // 1 inch
renderer.RenderingOptions.MarginBottom = 25.4;

// Or use the helper method for paper size
renderer.RenderingOptions.SetCustomPaperSizeInInches(8.5, 11);

// Telerik uses DIPs (device-independent pixels)
// IronPDF uses millimeters

// 1 inch = 25.4mm
renderer.RenderingOptions.MarginTop = 25.4;    // 1 inch
renderer.RenderingOptions.MarginBottom = 25.4;

// Or use the helper method for paper size
renderer.RenderingOptions.SetCustomPaperSizeInInches(8.5, 11);

' Telerik uses DIPs (device-independent pixels)
' IronPDF uses millimeters

' 1 inch = 25.4mm
renderer.RenderingOptions.MarginTop = 25.4    ' 1 inch
renderer.RenderingOptions.MarginBottom = 25.4

' Or use the helper method for paper size
renderer.RenderingOptions.SetCustomPaperSizeInInches(8.5, 11)

' Telerik uses DIPs (device-independent pixels)
' IronPDF uses millimeters

' 1 inch = 25.4mm
renderer.RenderingOptions.MarginTop = 25.4    ' 1 inch
renderer.RenderingOptions.MarginBottom = 25.4

' Or use the helper method for paper size
renderer.RenderingOptions.SetCustomPaperSizeInInches(8.5, 11)

Telerik Document Processing Migration Checklist

Pre-Migration Tasks

Audit your codebase to identify all Telerik Document Processing usage:

grep -r "using Telerik.Windows.Documents" --include="*.cs" .
grep -r "RadFlowDocument\|HtmlFormatProvider\|PdfFormatProvider" --include="*.cs" .

grep -r "using Telerik.Windows.Documents" --include="*.cs" .
grep -r "RadFlowDocument\|HtmlFormatProvider\|PdfFormatProvider" --include="*.cs" .

SHELL

Document existing implementations including format providers used, header/footer configurations, custom page settings, and any Flow Document model modifications.

During Migration

Remove Telerik NuGet packages
Install IronPDF NuGet package
Update using statements from Telerik.Windows.Documents to IronPdf
Add license key initialization at startup
Replace HtmlFormatProvider with ChromePdfRenderer
Remove RadFlowDocument intermediate steps
Replace PdfFormatProvider exports with direct SaveAs() calls
Update headers/footers to use HtmlHeaderFooter
Convert page settings to RenderingOptions
Update margin units from DIPs to millimeters

Post-Migration Verification

After migration, verify these improvements:

CSS rendering should improve significantly
Bootstrap layouts should render correctly (they don't in Telerik)
Flexbox and Grid layouts should work (they don't in Telerik)
JavaScript execution should function for dynamic content
Large document generation should complete without memory exceptions

Key Benefits of Migrating to IronPDF

Moving from Telerik Document Processing to IronPDF provides immediate advantages:

Modern Chromium Rendering Engine: IronPDF uses the same rendering engine as Google Chrome, ensuring PDFs render exactly as content appears in modern browsers. This eliminates the CSS compatibility issues inherent in Telerik's Flow Document model.

Full CSS3 and JavaScript Support: Flexbox, Grid, Bootstrap 5, CSS variables, and modern JavaScript all work correctly, ensuring compatibility with contemporary web standards.

Simpler API: Fewer lines of code, no intermediate document models, and intuitive method names reduce development time and maintenance burden.

Standalone Licensing: IronPDF offers straightforward licensing without requiring a comprehensive suite purchase, providing a cost-effective solution for teams that only need PDF capabilities.

Active Development: Regular updates ensure compatibility with current and future .NET versions, security patches, and feature enhancements.

Please note: Telerik is a registered trademark of its respective owner. This site is not affiliated with, endorsed by, or sponsored by Progress Software. All product names, logos, and brands are property of their respective owners. Comparisons are for informational purposes only and reflect publicly available information at the time of writing.

Curtis Chau

Chat with engineering team now

Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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