Digital signature

A digital signature is the cryptographic technology that proves a PDF is genuine and has not been altered. Using a certificate and a private key, it binds the signer's identity to the exact bytes of the document and seals them. If even a single character changes afterward, the signature breaks and the viewer flags it.

It answers two questions at once: who signed this, and has it been tampered with since. When you open a digitally signed PDF, the viewer checks the math against the signer's certificate and shows a green check if everything matches. This is what makes signed invoices, official records, and certified statements trustworthy.

It helps to separate two ideas. An electronic signature is the legal act of agreeing; a digital signature is the cryptographic mechanism, and it is what gives a signed PDF its tamper-evident integrity. The strongest e-signatures (like eIDAS QES) are built on top of digital signature technology.