Hi,
Thank you for using syncfusion product,
currently we do support to provide certificate as .pfx file, Internally we are using X509Certificate2 to create PdfCertificate form .pfx file.
PdfCertificate pdfCert = new PdfCertificate("Pdf.pfx", "123");
We have added the feature request "Need to add the support to instantiate a Pdfcertificate from X509Certificate2" in to our database, we will implement this any of our upcoming releases.
Please let us know if you need further assistance.
With Regards,
Praveen