For every website domain name that has been issued a TLS certificate, the website owner can digitally sign for verification purposes any PDF document using the same public key of already issued website TLS certificate.
This meams anyone can verify the authenticity of any digitally signed PDF by retrieving the public key of the website, just as they do now for a secure https connection.
No changes are required on the website!
Currently in development, Signature Bot for Let's Sign and Verify
Setup instructions are here