In .NET development when working with PDF documents, you may encounter a common issue: receiving a PDF with a digital signature and wanting to modify its content or adjust the layout, only to find it protected by the signature, which triggers errors during any operation.

The solution is straightforward: Digital signatures in PDFs are embedded as specific form field controls, and with C# and the free Free Spire.PDF library, you can remove all signatures with just a few lines of code.

Core Mechanism

A digital signature in a PDF is a special type of form field, represented by the class PdfSignatureFieldWidget. The main logic is to locate and remove this signature field, therreby removing the PDF's signature protection.

1. Load the PDF document with a digital signature
2. Retrieve all form fields in the document
3. Traverse the fields in reverse order to identify and delete signature-type fields
4. Save the modified PDF file
5. Release the document resources to avoid memory leaks

Complete Code

using Spire.Pdf;
using Spire.Pdf.Widget;

namespace RemoveSignature
{
    class Program
    {
        static void Main(string[] args)
        {
            // Load the PDF file to be processed
            PdfDocument pdf = new PdfDocument("C:\\Users\\Administrator\\Desktop\\Signature.pdf");

            // Get the collection of form fields in the document
            PdfFormWidget widgets = pdf.Form as PdfFormWidget;

            // Traverse in reverse order (to prevent index misalignment)
            for (int i = widgets.FieldsWidget.List.Count - 1; i >= 0; i--)
            {
                PdfFieldWidget widget = widgets.FieldsWidget.List[i] as PdfFieldWidget;

                // Check if the current field is a digital signature field
                if (widget is PdfSignatureFieldWidget)
                {
                    // Remove the signature
                    widgets.FieldsWidget.RemoveAt(i);
                    Console.WriteLine($"Removed signature field at index {i}");
                }
            }

            // Save the result
            pdf.SaveToFile("RemoveSignatures.pdf");
            Console.WriteLine("Processing complete!");

            // Remember to release resources
            pdf.Close();
        }
    }
}

Key Points Explained

1. Why Traverse in Reverse?

After calling List.RemoveAt(i), the indices of subsequent elements in the list will automatically shift forward. ✅ Traversing in forward order may skip the next element after deletion, leading to incomplete removal; ✅ Traversing in reverse order ensures no index misalignment, which is the standard way to delete elements from a collection.

2. Support for Removing All Signature Types

This method can remove:

• Visible signature images/stamps on the page
• Hidden electronic verification signatures in the background
• Single or multiple digital signatures

3. Free Spire.PDF Free Edition Limitations

The free edition has a PDF page limit (≤10 pages), which is sufficient for:

• Personal testing
• Small file processing
• Internal tool development for companies

Environment Setup (One Step)

Open the NuGet Package Manager Console in Visual Studio and run the following command to install the dependency library:

Install-Package FreeSpire.PDF

Or directly search for FreeSpire.PDF in the NuGet search bar and install it.

Important Notes

1. Always back up the original file The code generates a new file without overwriting the original, but it is recommended to manually back up important documents before making changes.
2. File path format For Windows paths, it is recommended to use the @ symbol to avoid errors caused by escape characetrs (e.g., @"C:\Test.pdf").
3. Legal Effect Reminder Digital signatures usually have legal effect. Ensure you have the right to modify/delete the file before proceeding, and do not perform any illegal operations.