How to Flatten PDFs in IronPDF C#

How Do I Flatten a PDF Document in C#?

Once IronPDF is installed, you can flatten PDF files with a single method call. The process works on existing PDF documents and on PDFs you render from HTML files before flattening.

The code example below uses the PdfDocument class to load an existing PDF. For dynamic PDF generation, render the form with the ChromePdfRenderer class before flattening.

To flatten a PDF file, call the Flatten method. This removes all interactive widgets including radio buttons, checkboxes, and text fields, making the document non-editable.

:path=/static-assets/pdf/content-code-examples/how-to/pdf-image-flatten-csharp-flatten-pdf.cs

using IronPdf;

// Select the desired PDF File
PdfDocument pdf = PdfDocument.FromFile("before.pdf");

// Flatten the pdf
pdf.Flatten();

// Save as a new file
pdf.SaveAs("after_flatten.pdf");

Imports IronPdf

' Select the desired PDF File
Private pdf As PdfDocument = PdfDocument.FromFile("before.pdf")

' Flatten the pdf
pdf.Flatten()

' Save as a new file
pdf.SaveAs("after_flatten.pdf")

:path=/static-assets/pdf/content-code-examples/how-to/pdf-image-flatten-csharp-3.cs

using IronPdf;

// Load a PDF with fillable forms
PdfDocument pdf = PdfDocument.FromFile("form-document.pdf");

// Optionally, pre-fill form fields by name before flattening
pdf.Form.FindFormField("name").Value = "John Doe";
pdf.Form.FindFormField("email").Value = "john@example.com";

// Flatten only pages 1-3, passing their zero-based indexes
pdf.Flatten(new[] { 0, 1, 2 });

// Save the result
pdf.SaveAs("flattened-form.pdf");

Imports IronPdf

' Load a PDF with fillable forms
Dim pdf As PdfDocument = PdfDocument.FromFile("form-document.pdf")

' Optionally, pre-fill form fields by name before flattening
pdf.Form.FindFormField("name").Value = "John Doe"
pdf.Form.FindFormField("email").Value = "john@example.com"

' Flatten only pages 1-3, passing their zero-based indexes
pdf.Flatten(New Integer() {0, 1, 2})

' Save the result
pdf.SaveAs("flattened-form.pdf")

How Can I Verify the PDF Is Flattened?

The output below shows the before and after states. The first PDF contains editable form fields. After the flatten method runs, the document becomes non-editable. This code works in any .NET project, including Blazor apps.

To verify successful flattening, check the form field count:

:path=/static-assets/pdf/content-code-examples/how-to/pdf-image-flatten-csharp-4.cs

using IronPdf;

// Load the flattened PDF
PdfDocument flattenedPdf = PdfDocument.FromFile("flattened.pdf");

// FormFieldCollection exposes Count directly on the Form property
if (flattenedPdf.Form.Count == 0)
{
    Console.WriteLine("PDF has been successfully flattened - no interactive fields remain.");
}
else
{
    Console.WriteLine($"Warning: {flattenedPdf.Form.Count} form fields still exist.");
}

Imports IronPdf

' Load the flattened PDF
Dim flattenedPdf As PdfDocument = PdfDocument.FromFile("flattened.pdf")

' FormFieldCollection exposes Count directly on the Form property
If flattenedPdf.Form.Count = 0 Then
    Console.WriteLine("PDF has been successfully flattened - no interactive fields remain.")
Else
    Console.WriteLine($"Warning: {flattenedPdf.Form.Count} form fields still exist.")
End If

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What Happens to Form Fields After Flattening?

When you flatten a PDF document, every interactive form element becomes part of the page content. Form fields turn into static text and graphics:

• Text fields become regular text on the page
• Checkboxes and radio buttons become static images showing their selected state
• Dropdown menus display only the selected value as plain text
• Digital signatures are preserved visually but lose cryptographic validation

This process is irreversible. Keep a copy of the original interactive PDF if you need future editing capabilities. Flattening keeps text and vector content selectable, unlike rasterizing pages to images, which converts each page into a flat picture. For documents requiring both security and editability, use PDF permissions and passwords instead of flattening.

When Should I Flatten My PDF Documents?

Common cases for flattening:

1. Legal Document Archival: Flatten contracts and agreements after signing to prevent content alteration and maintain legal integrity.

2. Report Distribution: Flatten financial reports and data sheets with calculated fields before distribution to prevent tampering.

3. Form Submission Processing: Create permanent records by flattening PDFs after users complete online forms.

4. Printing Optimization: Flattened PDFs print more reliably since printers don't process interactive elements.

5. File Size Reduction: Flattening can reduce file size by removing form field data structures, which pairs well with PDF compression.

Here's a batch processing example for archiving multiple completed forms:

using IronPdf;
using System.IO;

public class BatchPdfFlattener
{
    public static void FlattenAllPdfsInDirectory(string sourceDir, string outputDir)
    {
        // Ensure output directory exists
        Directory.CreateDirectory(outputDir);

        // Get all PDF files in source directory
        string[] pdfFiles = Directory.GetFiles(sourceDir, "*.pdf");

        foreach (string pdfFile in pdfFiles)
        {
            try
            {
                // Load the PDF
                PdfDocument pdf = PdfDocument.FromFile(pdfFile);

                // Flatten the document
                pdf.Flatten();

                // Save to output directory with "_flattened" suffix
                string fileName = Path.GetFileNameWithoutExtension(pdfFile);
                string outputPath = Path.Combine(outputDir, $"{fileName}_flattened.pdf");
                pdf.SaveAs(outputPath);

                Console.WriteLine($"Flattened: {fileName}");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error processing {pdfFile}: {ex.Message}");
            }
        }
    }
}

using IronPdf;
using System.IO;

public class BatchPdfFlattener
{
    public static void FlattenAllPdfsInDirectory(string sourceDir, string outputDir)
    {
        // Ensure output directory exists
        Directory.CreateDirectory(outputDir);

        // Get all PDF files in source directory
        string[] pdfFiles = Directory.GetFiles(sourceDir, "*.pdf");

        foreach (string pdfFile in pdfFiles)
        {
            try
            {
                // Load the PDF
                PdfDocument pdf = PdfDocument.FromFile(pdfFile);

                // Flatten the document
                pdf.Flatten();

                // Save to output directory with "_flattened" suffix
                string fileName = Path.GetFileNameWithoutExtension(pdfFile);
                string outputPath = Path.Combine(outputDir, $"{fileName}_flattened.pdf");
                pdf.SaveAs(outputPath);

                Console.WriteLine($"Flattened: {fileName}");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error processing {pdfFile}: {ex.Message}");
            }
        }
    }
}

Imports IronPdf
Imports System.IO

Public Class BatchPdfFlattener
    Public Shared Sub FlattenAllPdfsInDirectory(sourceDir As String, outputDir As String)
        ' Ensure output directory exists
        Directory.CreateDirectory(outputDir)

        ' Get all PDF files in source directory
        Dim pdfFiles As String() = Directory.GetFiles(sourceDir, "*.pdf")

        For Each pdfFile As String In pdfFiles
            Try
                ' Load the PDF
                Dim pdf As PdfDocument = PdfDocument.FromFile(pdfFile)

                ' Flatten the document
                pdf.Flatten()

                ' Save to output directory with "_flattened" suffix
                Dim fileName As String = Path.GetFileNameWithoutExtension(pdfFile)
                Dim outputPath As String = Path.Combine(outputDir, $"{fileName}_flattened.pdf")
                pdf.SaveAs(outputPath)

                Console.WriteLine($"Flattened: {fileName}")
            Catch ex As Exception
                Console.WriteLine($"Error processing {pdfFile}: {ex.Message}")
            End Try
        Next
    End Sub
End Class

Conclusion

Flattening converts a PDF's interactive form fields into static page content with a single Flatten() call, locking the document against further edits. Keep an unflattened copy if you might need to change the data later, since the operation cannot be undone.

If you need to populate fields first, set the form values before flattening; to assemble locked documents, merge or split them afterward.

Library Quick Access

Read More Documentation

Read the Documentation for more on how to flatten PDFs, edit and manipulate them, and more.

Visit IronPDF Documentation

Ready to see what else you can do? Check out our tutorial page here: Additional Features