What is a digital signature?
A digital signature is a cryptographic seal added to a file with a certificate, often called a digital ID. It identifies the signer and lets anyone check that the document hasn't been changed since it was signed: altering even one character of the signed file makes the check fail.
Also called: Certificate-based signature, Digital ID signature
Updated October 14, 2026 · By the PDFZyro team
How a digital signature works
The signer's certificate comes with a pair of keys: a private key only the signer controls, and a public key anyone can see. When they sign, the software calculates a fingerprint (a hash) of the document and signs that fingerprint with the private key. The signature and the certificate are stored inside the PDF. A PDF reader recalculates the fingerprint and checks the signature with the public key: if everything matches, the file is intact.
How far you can trust who signed depends on the certificate. One issued by a trusted certificate authority, after checking the person's identity, is recognized by PDF readers. A self-signed certificate still detects changes, but readers show its identity as not verified.
Digital vs electronic signatures
An electronic signature is the legal idea of signing electronically, whatever it looks like. A digital signature is the technology that makes a document tamper-evident. The two often work together: an e-signature service records each signer's intent and identity, then seals the finished PDF with a digital signature. In the EU, advanced and qualified electronic signatures under eIDAS are usually created as digital signatures.
Certification and timestamps
A PDF can carry several digital signatures, each covering the file as it was at that moment. The first can certify the document and say which changes are allowed afterwards, such as filling in form fields. A trusted timestamp records when the signature was made, independently of the signer's computer clock. Because any change breaks a signature, flatten or edit a document before it's signed, not after. Most PDF readers show the result of these checks in a signatures panel.
How to use digital signatures in PDFZyro
- PDFs you sign with Sign, and documents completed through signature requests, are sealed with a digital signature automatically, so any later change can be detected.
- To sign with your own certificate, create or upload a Digital ID in Settings, then open the document and choose Sign with a Digital ID.
- Open a signed PDF in PDFZyro to check its signatures: it shows who signed, when, the certificate, and whether the document changed afterwards.
Tools and guides
- Guide: How to sign a PDF for freeHow to sign a PDF online for free: create your signature, place it and download, other free ways to sign, getting others to sign, and what makes it valid.
- Sign PDFAdd your signature, initials and the date to any PDF.
- Request signaturesSend documents for e-signature with a full audit trail.
Related terms
- Electronic signature (e-signature)Any electronic sign that shows a person agrees to a document, from a typed name to a drawn signature.
- Certificate of completionA summary issued with a signed document showing who signed, when and how they were identified.
- PDF encryption (password protection)Scrambling a PDF so only people with the password can open it, or limiting printing and copying.