SelectPdf for .NET - Digital Signatures - C# / ASP.NET Sample Digital signatures can be added to a PDF document using PdfDigitalSignatureElement objects. Adding a digital signature requires a PKCS#12 certificate (provided as a .pfx or a .p12 file). Sample certificate file: Certificate The following sample shows how to add a digital signature to a pdf document using SelectPdf Library for .NET. IMPORTANT: Please note that the certificate used is a self signed certificate generated for testing purposes only. The signature will appear as not valid in a pdf viewer. Using a valid certificate will result in a valid digital signature.