Signature validity is determined by checking the authenticity of the signature’s digital ID certificate status and document integrity: In Acrobat or Reader, the signature of a certified or signed document is valid if you and the signer have a trust relationship.
The trust level of the certificate indicates the actions for which you trust the signer.
A digital ID is required to add the certifying digital signature.
Acrobat provides users with the capability to add a document timestamp to a PDF without also requiring an identity-based signature. (See Configure a timestamp server.) A timestamp assures the authenticity and existence of a document at a particular time.
For more information on trusting certificates, see About certificate-based signatures.
Long-term signature validation allows you to check the validity of a signature long after the document was signed.
Like PDF digital signatures, XML digital signatures ensure integrity, authentication, and non-repudiation in documents.
However, PDF signatures have multiple data verification states.
For more information about signature warnings and valid and invalid signatures, see the Each time a document is signed using a certificate, a signed version of the PDF at that time is saved with the PDF.
Each version is saved as append-only and the original cannot be modified.