Open the PDF in a PDF reader that supports digital signatures
Check whether the signature panel shows the document as signed
Verify that the signature status is marked as valid
Confirm that the signer’s certificate is trusted
Check that the certificate chain is complete and valid
Verify that the certificate is not expired
Verify that the certificate was not revoked
Confirm that the document has not been modified after signing
Check the signature timestamp if present
Validate the signature using a trusted validation tool or library
Compare the document hash against the signed hash
Ensure the signature algorithm is supported and secure
Confirm that the signing certificate matches the signer identity
Check for any validation warnings or errors reported by the PDF viewer
Revalidate after updating certificate trust stores and revocation data