Example of Digital Signature in ASP.NET Core PDF Library
This sample demonstrates how to digitally sign the PDF document with the PFX certificate file. It is possible to digitally sign with X509 certificates such as .pfx files with private keys and support for Hardware Security Module (HSM), Online Certificate Status Protocol (OCSP), Certificate Revocation List (CRL), and Windows Certificate Store to offer authenticity and integrity.
DEMO
SOURCE
It is also possible to digitally sign with timestamp.
More information about the digital signature and timestamp can be found in this documentation section.