MIGRATION GUIDES

How to Migrate from Spire.PDF to IronPDF in C#

Updated:May 18, 2026

Migrating from Spire.PDF to IronPDF moves your PDF generation off a legacy HTML path that frequently produces image-based text and onto a Chromium pipeline that produces true, selectable, searchable text. This guide provides a complete, step-by-step migration path covering Spire.PDF's HTML rendering trade-offs and reported font embedding issues.

Why Migrate from Spire.PDF to IronPDF

Understanding Spire.PDF

Spire.PDF is a robust, commercial PDF library designed for .NET developers to handle PDF documents efficiently. Spire.PDF has made its mark in the programming community for its specific capabilities, especially in legacy applications, and its integration capabilities seamlessly align with other components in the E-iceblue set of tools.

However, Spire.PDF has several fundamental problems that affect real-world usage, particularly around HTML-to-PDF conversion and modern web standards support.

Critical Technical Issues

Issue	Impact	IronPDF Solution
Text often rendered as images on the legacy HTML path	PDFs not searchable, not accessible, can't copy text	Real text rendering
IE/Edge Legacy or QtWebKit dependency on legacy path	Outdated rendering, modern CSS issues	Bundled modern Chromium
Font embedding issues reported	Documents look wrong on other systems	Reliable font handling
Large deployment footprint	Higher memory usage	Bundled Chromium runtime
Limited CSS on legacy path	Modern layouts don't render correctly	Full CSS3 support

The Core Problem: Image-Based PDFs

One reported drawback of Spire.PDF's legacy HTML path is its inclination to render text within HTML documents as images. This produces PDFs where text is not selectable or searchable — a limitation for applications needing search or text interaction.

When you use Spire.PDF's LoadFromHTML() method on the legacy engine, it often renders text as bitmap images rather than actual text, creating these problems:

Text CANNOT be selected
Text CANNOT be searched
Text CANNOT be copied
Screen readers CANNOT read it (accessibility violation)
File size is MUCH larger
Zooming causes pixelation

Spire.PDF vs IronPDF Comparison

Feature	Spire.PDF	IronPDF
HTML to PDF Rendering	Legacy IE/QtWebKit by default; opt-in `ChromeHtmlConverter` (requires system Chrome) since v10.7.21	Bundled Chromium (modern, no extra install)
Text Output (HTML path)	Often rendered as images on legacy path	Real text (selectable and searchable)
Font Handling	Issues reported on legacy path	Reliable font handling
CSS3 Support	Limited on legacy path	Full
Flexbox/Grid	Not supported on legacy path	Supported
JavaScript	Limited on legacy path	Full ES6+
PDF Accessibility	Poor on legacy path (image-based output)	Tagged-text output
API Design	Many overloads, manual page settings	Single fluent renderer
Licensing	Freemium/Commercial	Commercial

On modern .NET, IronPDF addresses Spire.PDF's HTML-to-PDF trade-offs by rendering text as actual selectable text rather than images, so the resulting PDFs are searchable and accessible.

Before You Start

Prerequisites

.NET Environment: .NET Framework 4.6.2+ or .NET Core 3.1+ / .NET 5/6/7/8/9+
NuGet Access: Ability to install NuGet packages
IronPDF License: Obtain your license key from ironpdf.com

NuGet Package Changes

# Remove Spire.PDF
dotnet remove package Spire.PDF
dotnet remove package FreeSpire.PDF  # If using free version

# Install IronPDF
dotnet add package IronPdf

# Remove Spire.PDF
dotnet remove package Spire.PDF
dotnet remove package FreeSpire.PDF  # If using free version

# Install IronPDF
dotnet add package IronPdf

SHELL

License Configuration

// 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"

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Complete API Reference

Namespace Changes

// Before: Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using Spire.Pdf.HtmlConverter;

// After: IronPDF
using IronPdf;
using IronPdf.Editing;

// Before: Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using Spire.Pdf.HtmlConverter;

// After: IronPDF
using IronPdf;
using IronPdf.Editing;

Imports IronPdf
Imports IronPdf.Editing

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Core API Mappings

Spire.PDF	IronPDF
`new PdfDocument()`	`new ChromePdfRenderer()`
`pdf.LoadFromHTML()`	`renderer.RenderHtmlAsPdf()`
`pdf.LoadFromFile()`	`PdfDocument.FromFile()`
`pdf.SaveToFile()`	`pdf.SaveAs()`
`pdf.Close()`	Not needed
`pdf.Pages.Add()`	`renderer.RenderHtmlAsPdf()`
`pdf.InsertPageRange()`	`PdfDocument.Merge()`
`page.Canvas.DrawString()`	`TextStamper` + `ApplyStamp()`
`PdfFont`	CSS styling in HTML
`PdfBrush`	CSS styling in HTML

Code Migration Examples

Example 1: HTML to PDF Conversion

Before (Spire.PDF):

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfHtmlLayoutFormat htmlLayoutFormat = new PdfHtmlLayoutFormat();

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

        pdf.LoadFromHTML(htmlString, false, true, true);
        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfHtmlLayoutFormat htmlLayoutFormat = new PdfHtmlLayoutFormat();

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

        pdf.LoadFromHTML(htmlString, false, true, true);
        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

Imports Spire.Pdf
Imports Spire.Pdf.Graphics
Imports System

Class Program
    Shared Sub Main()
        Dim pdf As New PdfDocument()
        Dim htmlLayoutFormat As New PdfHtmlLayoutFormat()

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

        pdf.LoadFromHTML(htmlString, False, True, True)
        pdf.SaveToFile("output.pdf")
        pdf.Close()
    End Sub
End Class

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After (IronPDF):

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();

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

        var pdf = renderer.RenderHtmlAsPdf(htmlString);
        pdf.SaveAs("output.pdf");
    }
}

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();

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

        var pdf = renderer.RenderHtmlAsPdf(htmlString);
        pdf.SaveAs("output.pdf");
    }
}

Imports IronPdf
Imports System

Class Program
    Shared Sub Main()
        Dim renderer = New ChromePdfRenderer()

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

        Dim pdf = renderer.RenderHtmlAsPdf(htmlString)
        pdf.SaveAs("output.pdf")
    End Sub
End Class

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This example demonstrates the fundamental difference in HTML rendering. Spire.PDF uses LoadFromHTML() with a PdfHtmlLayoutFormat object, which often renders text as bitmap images. The result is PDFs where users cannot select, copy, or search text.

IronPDF uses a ChromePdfRenderer with RenderHtmlAsPdf(), producing true text that is fully selectable, searchable, and accessible. No Close() call is required—IronPDF uses the dispose pattern for automatic cleanup. See the HTML to PDF documentation for comprehensive examples.

Example 2: Merging Multiple PDFs

Before (Spire.PDF):

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf1 = new PdfDocument();
        pdf1.LoadFromFile("document1.pdf");

        PdfDocument pdf2 = new PdfDocument();
        pdf2.LoadFromFile("document2.pdf");

        pdf1.InsertPageRange(pdf2, 0, pdf2.Pages.Count - 1);

        pdf1.SaveToFile("merged.pdf");
        pdf1.Close();
        pdf2.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf1 = new PdfDocument();
        pdf1.LoadFromFile("document1.pdf");

        PdfDocument pdf2 = new PdfDocument();
        pdf2.LoadFromFile("document2.pdf");

        pdf1.InsertPageRange(pdf2, 0, pdf2.Pages.Count - 1);

        pdf1.SaveToFile("merged.pdf");
        pdf1.Close();
        pdf2.Close();
    }
}

Imports Spire.Pdf
Imports System

Class Program
    Shared Sub Main()
        Dim pdf1 As New PdfDocument()
        pdf1.LoadFromFile("document1.pdf")

        Dim pdf2 As New PdfDocument()
        pdf2.LoadFromFile("document2.pdf")

        pdf1.InsertPageRange(pdf2, 0, pdf2.Pages.Count - 1)

        pdf1.SaveToFile("merged.pdf")
        pdf1.Close()
        pdf2.Close()
    End Sub
End Class

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After (IronPDF):

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        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;
using System;

class Program
{
    static void Main()
    {
        var pdf1 = PdfDocument.FromFile("document1.pdf");
        var pdf2 = PdfDocument.FromFile("document2.pdf");

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

        merged.SaveAs("merged.pdf");
    }
}

Imports IronPdf
Imports System

Class Program
    Shared Sub Main()
        Dim pdf1 = PdfDocument.FromFile("document1.pdf")
        Dim pdf2 = PdfDocument.FromFile("document2.pdf")

        Dim merged = PdfDocument.Merge(pdf1, pdf2)

        merged.SaveAs("merged.pdf")
    End Sub
End Class

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Spire.PDF requires manually loading each document with new PdfDocument() + LoadFromFile(), then using InsertPageRange() to specify which pages to insert, and finally calling Close() on each document.

IronPDF uses the simpler PdfDocument.FromFile() pattern and a static PdfDocument.Merge() method that accepts multiple documents. No Close() calls needed. Learn more in our tutorials.

Example 3: Adding Text to a PDF

Before (Spire.PDF):

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfPageBase page = pdf.Pages.Add();

        PdfFont font = new PdfFont(PdfFontFamily.Helvetica, 20);
        PdfBrush brush = new PdfSolidBrush(Color.Black);

        page.Canvas.DrawString("Hello from Spire.PDF!", font, brush, new PointF(50, 50));

        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfPageBase page = pdf.Pages.Add();

        PdfFont font = new PdfFont(PdfFontFamily.Helvetica, 20);
        PdfBrush brush = new PdfSolidBrush(Color.Black);

        page.Canvas.DrawString("Hello from Spire.PDF!", font, brush, new PointF(50, 50));

        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

Imports Spire.Pdf
Imports Spire.Pdf.Graphics
Imports System.Drawing
Imports System

Class Program
    Shared Sub Main()
        Dim pdf As New PdfDocument()
        Dim page As PdfPageBase = pdf.Pages.Add()

        Dim font As New PdfFont(PdfFontFamily.Helvetica, 20)
        Dim brush As PdfBrush = New PdfSolidBrush(Color.Black)

        page.Canvas.DrawString("Hello from Spire.PDF!", font, brush, New PointF(50, 50))

        pdf.SaveToFile("output.pdf")
        pdf.Close()
    End Sub
End Class

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After (IronPDF):

// NuGet: Install-Package IronPdf
using IronPdf;
using IronPdf.Editing;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();
        var pdf = renderer.RenderHtmlAsPdf("<html><body></body></html>");

        var textStamper = new TextStamper()
        {
            Text = "Hello from IronPDF!",
            FontSize = 20,
            VerticalOffset = 50,
            HorizontalOffset = 50
        };

        pdf.ApplyStamp(textStamper);
        pdf.SaveAs("output.pdf");
    }
}

// NuGet: Install-Package IronPdf
using IronPdf;
using IronPdf.Editing;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();
        var pdf = renderer.RenderHtmlAsPdf("<html><body></body></html>");

        var textStamper = new TextStamper()
        {
            Text = "Hello from IronPDF!",
            FontSize = 20,
            VerticalOffset = 50,
            HorizontalOffset = 50
        };

        pdf.ApplyStamp(textStamper);
        pdf.SaveAs("output.pdf");
    }
}

Imports IronPdf
Imports IronPdf.Editing
Imports System

Class Program
    Shared Sub Main()
        Dim renderer = New ChromePdfRenderer()
        Dim pdf = renderer.RenderHtmlAsPdf("<html><body></body></html>")

        Dim textStamper = New TextStamper() With {
            .Text = "Hello from IronPDF!",
            .FontSize = 20,
            .VerticalOffset = 50,
            .HorizontalOffset = 50
        }

        pdf.ApplyStamp(textStamper)
        pdf.SaveAs("output.pdf")
    End Sub
End Class

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Spire.PDF uses a canvas-based drawing model with PdfFont, PdfBrush, and page.Canvas.DrawString() to position text at specific coordinates using PointF.

IronPDF uses a TextStamper object with intuitive properties like Text, FontSize, VerticalOffset, and HorizontalOffset, then applies it with ApplyStamp(). This approach is more declarative and easier to maintain.

The Text-as-Images Problem

Why This Is Critical

When Spire.PDF converts HTML to PDF using image-based rendering, your documents lose essential functionality:

1. No Text Search: Users cannot use Ctrl+F to find text. Document management systems cannot index content.

2. No Text Selection/Copy: Users trying to copy a quote, reference, or data cannot select text—it's an image.

3. Accessibility Violations: Image-based PDFs fail WCAG 2.1 compliance, Section 508 compliance (US Government), ADA requirements, and screen reader compatibility.

4. Large File Sizes: Image-based output from the legacy HTML path can produce files an order of magnitude larger than the text-based output IronPDF produces for the same content.

Detection: Is Your PDF Image-Based?

Open your Spire.PDF-generated document and try these tests:

Text Selection: Click and drag over text. If nothing highlights → IMAGE-BASED
Ctrl+F Search: Search for any word on the page. If "No matches found" → IMAGE-BASED
Copy/Paste: Select and copy text to notepad. If nothing pastes → IMAGE-BASED

The Legacy HTML Engine Problem

Spire.PDF's Rendering Engines

Spire.PDF's LoadFromHTML(...) historically offered two paths: a non-plugin path backed by the Windows Internet Explorer / Edge Legacy WebBrowser control, and a separately-downloaded QT/WebKit plugin path. Spire.PDF 10.7.21 (mid-2024) added a third option, ChromeHtmlConverter, which shells out to a locally installed Google Chrome — but it is opt-in and the original LoadFromHTML API still defaults to the legacy engines. IE/Edge Legacy was deprecated by Microsoft in 2022, modern CSS is unreliable on the legacy path, JavaScript support is partial, and rendering can vary across systems.

Modern CSS That Fails in Spire.PDF


<div style="display: flex; justify-content: space-between; gap: 20px;">
    <div style="flex: 1;">Column 1</div>
    <div style="flex: 1;">Column 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px;">
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>

<style>
:root { --primary-color: #007bff; }
h1 { color: var(--primary-color); }
</style>


<div style="display: flex; justify-content: space-between; gap: 20px;">
    <div style="flex: 1;">Column 1</div>
    <div style="flex: 1;">Column 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px;">
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>

<style>
:root { --primary-color: #007bff; }
h1 { color: var(--primary-color); }
</style>

HTML

IronPDF uses modern Chromium rendering, so all these CSS features work correctly.

New Capabilities After Migration

After migrating to IronPDF, you gain capabilities that Spire.PDF cannot provide:

Selectable, Searchable Text

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Important Contract</h1>");
pdf.SaveAs("contract.pdf");

// Result:
// ✅ Text is fully selectable
// ✅ Text is searchable with Ctrl+F
// ✅ Text can be copied to clipboard
// ✅ Screen readers work perfectly
// ✅ File size is compact
// ✅ Zooming is crystal clear

var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Important Contract</h1>");
pdf.SaveAs("contract.pdf");

// Result:
// ✅ Text is fully selectable
// ✅ Text is searchable with Ctrl+F
// ✅ Text can be copied to clipboard
// ✅ Screen readers work perfectly
// ✅ File size is compact
// ✅ Zooming is crystal clear

Dim renderer = New ChromePdfRenderer()
Dim pdf = renderer.RenderHtmlAsPdf("<h1>Important Contract</h1>")
pdf.SaveAs("contract.pdf")

' Result:
' ✅ Text is fully selectable
' ✅ Text is searchable with Ctrl+F
' ✅ Text can be copied to clipboard
' ✅ Screen readers work perfectly
' ✅ File size is compact
' ✅ Zooming is crystal clear

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Modern CSS Support

var renderer = new ChromePdfRenderer();

var html = @"
<style>
    :root { --primary: #007bff; }
    .container { display: flex; gap: 20px; }
    .grid { display: grid; grid-template-columns: repeat(3, 1fr); }
</style>
<div class='container'>
    <div style='flex: 1; color: var(--primary)'>Column 1</div>
    <div style='flex: 1'>Column 2</div>
</div>
<div class='grid'>
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>";

var pdf = renderer.RenderHtmlAsPdf(html);
// All modern CSS features render correctly!

var renderer = new ChromePdfRenderer();

var html = @"
<style>
    :root { --primary: #007bff; }
    .container { display: flex; gap: 20px; }
    .grid { display: grid; grid-template-columns: repeat(3, 1fr); }
</style>
<div class='container'>
    <div style='flex: 1; color: var(--primary)'>Column 1</div>
    <div style='flex: 1'>Column 2</div>
</div>
<div class='grid'>
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>";

var pdf = renderer.RenderHtmlAsPdf(html);
// All modern CSS features render correctly!

Dim renderer = New ChromePdfRenderer()

Dim html = "
<style>
    :root { --primary: #007bff; }
    .container { display: flex; gap: 20px; }
    .grid { display: grid; grid-template-columns: repeat(3, 1fr); }
</style>
<div class='container'>
    <div style='flex: 1; color: var(--primary)'>Column 1</div>
    <div style='flex: 1'>Column 2</div>
</div>
<div class='grid'>
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>"

Dim pdf = renderer.RenderHtmlAsPdf(html)
' All modern CSS features render correctly!

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HTML-Based Watermarks

var pdf = renderer.RenderHtmlAsPdf(html);
pdf.ApplyWatermark(@"
    <div style='
        font-size: 48px;
        color: rgba(255, 0, 0, 0.5);
        transform: rotate(-45deg);
    '>DRAFT</div>");

var pdf = renderer.RenderHtmlAsPdf(html);
pdf.ApplyWatermark(@"
    <div style='
        font-size: 48px;
        color: rgba(255, 0, 0, 0.5);
        transform: rotate(-45deg);
    '>DRAFT</div>");

Dim pdf = renderer.RenderHtmlAsPdf(html)
pdf.ApplyWatermark("
    <div style='
        font-size: 48px;
        color: rgba(255, 0, 0, 0.5);
        transform: rotate(-45deg);
    '>DRAFT</div>")

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Migration Checklist

Pre-Migration

Inventory all Spire.PDF usages in codebase
Test existing PDFs for text selectability (critical issue detection)
Document LoadFromHTML() calls (these are the priority to fix)
Obtain IronPDF license key from ironpdf.com

Code Updates

Remove Spire.PDF NuGet package (and FreeSpire.PDF if using free version)
Install IronPdf NuGet package
Update namespace imports (using Spire.Pdf; → using IronPdf;)
Replace LoadFromHTML() with RenderHtmlAsPdf() (CRITICAL FIX)
Replace new PdfDocument() + LoadFromFile() with PdfDocument.FromFile()
Replace InsertPageRange() with PdfDocument.Merge()
Replace Canvas.DrawString() with TextStamper + ApplyStamp()
Replace SaveToFile() with SaveAs()
Remove all Close() calls (not needed in IronPDF)
Add license initialization at application startup

Testing

Verify text is selectable in generated PDFs (CRITICAL TEST)
Verify CSS rendering improvements (Flexbox/Grid now work)
Verify file sizes are smaller
Test accessibility with screen readers
Performance comparison

Please noteSpire is a registered trademark of its respective owner. This site is not affiliated with, endorsed by, or sponsored by e-iceblue. 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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How to Migrate from Spire.PDF to IronPDF in C#

Why Migrate from Spire.PDF to IronPDF

Understanding Spire.PDF

Critical Technical Issues

The Core Problem: Image-Based PDFs

Spire.PDF vs IronPDF Comparison

Before You Start

Prerequisites

NuGet Package Changes

License Configuration

Complete API Reference

Namespace Changes

Core API Mappings

Code Migration Examples

Example 1: HTML to PDF Conversion

Example 2: Merging Multiple PDFs

Example 3: Adding Text to a PDF

The Text-as-Images Problem

Why This Is Critical

Detection: Is Your PDF Image-Based?

The Legacy HTML Engine Problem

Spire.PDF's Rendering Engines

Modern CSS That Fails in Spire.PDF

New Capabilities After Migration

Selectable, Searchable Text

Modern CSS Support

HTML-Based Watermarks

Migration Checklist

Pre-Migration

Code Updates

Testing

Iron Support Team

Start Free 30 Day Trial

How to Migrate from Spire.PDF to IronPDF in C#

Why Migrate from Spire.PDF to IronPDF

Understanding Spire.PDF

Critical Technical Issues

The Core Problem: Image-Based PDFs

Spire.PDF vs IronPDF Comparison

Before You Start

Prerequisites

NuGet Package Changes

License Configuration

Complete API Reference

Namespace Changes

Core API Mappings

Code Migration Examples

Example 1: HTML to PDF Conversion

Example 2: Merging Multiple PDFs

Example 3: Adding Text to a PDF

The Text-as-Images Problem

Why This Is Critical

Detection: Is Your PDF Image-Based?

The Legacy HTML Engine Problem

Spire.PDF's Rendering Engines

Modern CSS That Fails in Spire.PDF

New Capabilities After Migration

Selectable, Searchable Text

Modern CSS Support

HTML-Based Watermarks

Migration Checklist

Pre-Migration

Code Updates

Testing

Related Articles

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How to Migrate from Scryber.Core to IronPDF in C#

How to Migrate from XFINIUM.PDF to IronPDF in C#

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