Here is C# code that demonstrates how to remove digital signature from PDF document: 
using Vintasoft.Imaging.Pdf;
using Vintasoft.Imaging.Pdf.Tree.InteractiveForms;

public class PdfSignaturesHelper2
{
    /// <summary>
    /// Removes all digital signatures from PDF document.
    /// </summary>
    /// <param name="pdfFilename">The filename of PDF document.</param>
    public static void RemoveDigitalSignaturesFromPdfDocument(string pdfFilename)
    {
        // open PDF document
        using (PdfDocument document = new PdfDocument(pdfFilename))
        {
            // if PDF document has PDF interactive form
            if (document.InteractiveForm != null)
            {
                // for each signature field
                foreach (PdfInteractiveFormField field in
                    document.InteractiveForm.GetSignatureFields())
                {
                    // remove signature field
                    field.Remove();
                }

                // pack PDF document
                document.Pack();
            }
        }
    }
}