PAdES (PDF Advanced Electronic Signatures) is an ETSI standard that solves the problem of PDF signatures becoming unverifiable after certificate expiry. It defines four progressive levels: B-B (basic signature), B-T (adds RFC 3161 trusted timestamp), B-LT (embeds OCSP/CRL revocation data and certificate chain for offline verification), and B-LTA (adds archival document-level timestamp for multi-decade retention). The guide walks through implementing each level in .NET using the Syncfusion PDF Library, with code samples for loading PFX certificates, applying CAdES cryptographic standard, connecting to a TSA, enabling LTV, and adding archival timestamps. A comparison table helps developers choose the right level based on document retention needs.
