Flatten PDF in C#

Flattening a PDF means turning interactive PDF content, such as form fields, into fixed page content. The output PDF is easier to share, print, archive, or send to users who should not edit form values after the file is created.

Aspose.HTML for .NET flattens form fields when creating a PDF from HTML or MHTML. Create PdfSaveOptions, set FormFieldBehaviour = FormFieldBehaviour.Flattened, and call Converter.ConvertHTML() or Converter.ConvertMHTML() to save a fixed PDF file.

What Does It Mean to Flatten a PDF?

To flatten a PDF file is to make interactive elements part of the static page appearance. In this article, the important case is form fields: text boxes, checkboxes, and other form controls are written into the PDF as non-interactive content instead of remaining editable fields.

This is useful when a generated PDF should preserve the final visual result and not behave like a fillable form. It can also improve compatibility with PDF viewers, printers, document exchange systems, and archive workflows.

When to Flatten a PDF

Flatten a PDF when the output should be fixed after conversion:

• Distribution – send a completed document without editable form fields.
• Printing – reduce viewer-specific behavior for form controls during printing.
• Archiving – store a stable document state that does not require user interaction.
• Review workflows – prevent accidental edits after a form-based HTML page is converted to PDF.
• Compatibility – make the PDF appearance less dependent on a reader’s form-field rendering behavior.

Flatten HTML to PDF in C#

To flatten a PDF during HTML to PDF conversion, set the FormFieldBehaviour property on PdfSaveOptions. The default value is FormFieldBehaviour.Interactive, so form fields remain editable unless you explicitly set FormFieldBehaviour.Flattened.

To convert HTML to a flattened PDF:

1. Load the source HTML document with HTMLDocument.
2. Create a PdfSaveOptions instance.
3. Set options.FormFieldBehaviour = FormFieldBehaviour.Flattened.
4. Call Converter.ConvertHTML() with the document, options, and output path.
5. Save the resulting PDF with form fields flattened into static content.

// Flatten PDF during HTML to PDF conversion using C#

// Prepare a path to an HTML source file
string sourcePath = Path.Combine(DataDir, "SampleHtmlForm.html");

// Initialize an HTML document from the file
using HTMLDocument document = new HTMLDocument(sourcePath);

// Prepare PDF save options
PdfSaveOptions options = new PdfSaveOptions
{
    // Flatten all form fields
    FormFieldBehaviour = FormFieldBehaviour.Flattened
};

// Prepare a path to the result file
string resultPath = Path.Combine(OutputDir, "form-flattened.pdf");

// Convert HTML to PDF
Converter.ConvertHTML(document, options, resultPath);

Flatten MHTML to PDF in C#

MHTML can also contain HTML forms and related resources in one archive file. The flattening approach is the same: create PdfSaveOptions, set FormFieldBehaviour.Flattened, and pass the options to Converter.ConvertMHTML().

To convert MHTML to a flattened PDF:

1. Open the source MHTML file as a stream.
2. Create a PdfSaveOptions instance.
3. Set options.FormFieldBehaviour = FormFieldBehaviour.Flattened.
4. Call Converter.ConvertMHTML(stream, options, outputPath).
5. Save the PDF with non-interactive form-field output.

// Flatten PDF during MHTML to PDF conversion using C#

// Prepare a path to an MHTML source file
string sourcePath = Path.Combine(DataDir, "SampleHtmlForm.mhtml");

// Initialize PDF save options
PdfSaveOptions options = new PdfSaveOptions
{
    // Flatten all form fields
    FormFieldBehaviour = FormFieldBehaviour.Flattened
};

// Prepare a path to the result file
string resultPath = Path.Combine(OutputDir, "document-flattened.pdf");

// Convert MHTML to PDF
Converter.ConvertMHTML(sourcePath, options, resultPath);

FormFieldBehaviour Values

FormFieldBehaviour controls what happens to form fields when Aspose.HTML creates the output PDF. Use Flattened when the PDF should preserve the final appearance, and use Interactive when the PDF should remain fillable.

Value	Result in the output PDF	Use it when
FormFieldBehaviour.Flattened	Form fields are merged into static page content and are no longer editable.	The PDF is final and should be shared, printed, or archived.
FormFieldBehaviour.Interactive	Form fields remain interactive and can be filled or edited in a PDF viewer. This is the default value.	Users should complete or edit the form after conversion.

Common PDF Flattening Mistakes

Mistake	How to fix it
Expecting Aspose.HTML to flatten an existing arbitrary PDF file	Use this workflow when generating PDF from HTML or MHTML. Aspose.HTML controls flattening during conversion to PDF.
Forgetting to set FormFieldBehaviour	Set options.FormFieldBehaviour = FormFieldBehaviour.Flattened; the default value is Interactive.
Flattening a PDF that should remain fillable	Use FormFieldBehaviour.Interactive when users need to complete form fields after conversion.
Assuming CSS alone controls PDF form behavior	Use PdfSaveOptions.FormFieldBehaviour for PDF form-field behavior; CSS controls visual styling, not interactivity in the output PDF.
Mixing HTML and MHTML conversion APIs	Use Converter.ConvertHTML() for HTML documents and Converter.ConvertMHTML() for MHTML streams.

Related PDF Workflows

• Convert HTML to PDF in C# – create PDF files from HTML with Converter.ConvertHTML() and PdfSaveOptions.
• Convert MHTML to PDF in C# – convert MHTML streams to PDF with Converter.ConvertMHTML().
• Resize Document During Conversion – adjust page size and margins when creating PDF output.
• Rendering Options in C# – review lower-level PDF rendering options, including FormFieldBehaviour.

The complete C# examples and data files are available in the Aspose.HTML for .NET GitHub repository. You can also test HTML to PDF output with the online HTML to PDF Converter.