Digital Signatures in PDF: How They Work

Digital signatures provide a way to verify the authenticity and integrity of a PDF document. They confirm who signed the document and whether it has been modified since signing.

How Digital Signatures Work

Digital signatures use public key cryptography. The signer has a private key (kept secret) and a public key (shared freely). When signing a document, the signer’s software creates a hash of the document content and encrypts it with the private key. Anyone with the public key can verify the signature.

Types of PDF Signatures

Certification Signatures

Applied by the document author, certification signatures can specify what changes are allowed after signing. The author might allow form filling but prevent any other modifications.

Approval Signatures

These are added by reviewers or approvers. Multiple people can add approval signatures to the same document, creating a chain of approvals.

Certificate Authorities

For signatures to be trusted, the signer’s certificate should be issued by a recognized Certificate Authority (CA). Self-signed certificates work for internal use but may not be trusted by external recipients.

Legal Validity

Digital signatures on PDFs are legally binding in most countries, including under the US ESIGN Act, European eIDAS regulation, and similar laws worldwide. They provide stronger legal standing than scanned handwritten signatures because they verify both identity and document integrity.

Best Practices

Sign the final version of the document only. Add signatures after all editing is complete. Use certificates from recognized CAs for external documents. Keep your private key secure and never share it. Verify signatures when receiving signed documents.