What Is a Digital Signature? Digital vs Electronic vs Wet Signatures

A digital signature is a cryptographic seal that proves who signed a document and that the document has not been changed since it was signed. It is different from simply placing a picture of your handwriting on a page – and knowing the difference tells you which one a contract, bank or government office actually needs.

Digital signature definition

A digital signature uses public-key cryptography. The signer has a private key, usually protected by a certificate issued by a trusted certificate authority (CA). When the document is signed, software calculates a unique fingerprint (hash) of the file and encrypts it with the private key. Anyone can then use the signer's public certificate to check that the fingerprint still matches the file.

If even one character changes after signing, the check fails – that is what makes a digital signature tamper-evident.

Digital signature vs electronic signature

• Electronic signature (e-signature): any electronic sign of agreement – a typed name, a checkbox, or an image of your handwritten signature placed on a PDF. Quick and widely accepted for everyday documents.
• Digital signature: a specific, certificate-based type of electronic signature with cryptographic proof of identity and integrity.

Many laws, such as eIDAS in the EU and the ESIGN Act in the US, recognise electronic signatures, while some processes (court filings, tenders, certain tax or real-estate documents) require a certificate-based or qualified signature.

What is a wet signature?

A wet signature is a signature written by hand in ink on paper. Some organisations still ask for one – for example for certain notarised or government documents. When a “wet signature” is required, an e-signature or scanned image may not be accepted, so check the instructions first.

How to verify a digital signature in a PDF

• Open the PDF in Adobe Acrobat Reader (free) and open the Signature panel.
• A valid signature shows the signer's name, the certificate issuer and a message that the document has not been modified since signing.
• Warnings such as “validity unknown” usually mean the signer's certificate is not in your trusted list; “document has been altered” means the file changed after signing.

Note: a signature image placed with an e-signature tool will not appear in the Signature panel, because it is not a cryptographic signature.

When is an electronic signature enough?

For internal approvals, quotes, NDAs, rental forms, school permission slips and most HR paperwork, a simple electronic signature is usually fine. For legally sensitive filings, ask the recipient which type they accept.

How to add an electronic signature to a PDF

With Sign PDF you can draw your signature with a finger or mouse, or upload an image of it, then place it on any page. Everything happens in your browser – the document and the signature are never uploaded. Remember that this is an electronic (visual) signature, not a certificate-based digital signature.

Tools mentioned in this guide