Is this PDF digitally signed? Check before you countersign

A picture of a signature versus a digital signature

"Signed PDF" covers two very different things. Most signed PDFs people receive carry a picture: a name drawn with a finger or mouse, typed in a handwriting font, or scanned and pasted onto the page. That picture is part of the page content like any logo or photo. Adding your own signature further down the page doesn't touch it.

A digital signature works differently. The signer's software calculates a fingerprint of the file's exact bytes, encrypts it with the signer's private key and stores it inside the PDF together with the signer's certificate. Adobe's technical guide to PDF signatures describes both approval and certification signatures as covering the byte range of the whole file. Change a single byte inside that range and the fingerprint no longer matches, which is the whole point: anyone can see that the document was altered after signing.

The other party doesn't need a digital ID of their own for this. According to their own documentation, Docusign seals every completed envelope and Acrobat Sign certifies the final agreement PDF. The name on the page was typed or drawn; the file around it is sealed.

Why adding your signature can break theirs

The PDF format allows a later signer to add something without disturbing earlier signatures: the new content is appended to the end of the file as an "incremental update", and the original bytes stay exactly as they were. That is how a second certificate-based signature is added in apps like Adobe Acrobat, and it only works if the author's certification permits signing and there is a field to sign.

Many PDF editors — this site's signer included — don't append. They place your signature on the page and write a fresh copy of the whole file. For a picture-signed contract that's exactly what you want. For a digitally signed one, the fresh copy no longer matches the fingerprint, so a PDF reader that checks signatures reports theirs as invalid. Their signature still looks identical on screen; what's lost is the proof, in your copy, that nothing changed after they signed.

The checker tests this directly: for every signature it compares the byte range recorded at signing time with the file in front of it, so it can also tell you when a PDF has already been re-saved and its signature is probably broken before you even start.

How to read the result

No digital signature found: sign here without worry. The other party's signature is a picture on the page and stays where it is. The checker can't recognise drawn or pasted signatures — to software they're just images — so look at the pages yourself to confirm they actually signed.

Empty signature field: nobody has digitally signed yet, but the sender prepared a field for a certificate-based signature. Signing here breaks nothing; the field simply stays empty. If the sender explicitly asked for a digital-ID signature in that field, use an app that supports certificate-based signing instead.

Digitally signed: at least one person or service sealed the file. Before you sign a copy here, choose one of the routes below.

Certified: the author certified the document and chose what later changes are allowed. "No changes allowed" means any addition breaks the certification, in every app. "Form fill-in and signing" means an appended certificate-based signature in an existing field is fine, but a signature drawn onto the page and saved as a new file is not.

Your options when the PDF is digitally signed

1. Sign in the same system. If the contract came from DocuSign, Adobe Acrobat Sign or a similar service, ask the sender to route it to you there as the next signer. Your signature is then added without breaking the seal.

2. Use the empty signature field. If the checker lists an unsigned field for you and the certification allows signing, sign that field with a digital ID in Adobe Acrobat or another app that supports certificate-based signing.

3. Sign a separate counterpart. Sign your own copy of the final text, or just its signature page if the contract has a counterparts clause, and send it back together with their untouched original.

4. Agree that a picture signature is enough. Many ordinary agreements are signed that way. Say so in your reply, keep their digitally signed file unchanged as the record of what they signed, and countersign a copy here.

Privacy and limits

The checker runs entirely in your browser and forgets the file when you close the tab. It reads the file's structure but doesn't verify certificates, revocation or whether a signer is trusted — for that, use the Signatures panel in Adobe Acrobat Reader.

It also flags encrypted PDFs. The signer on this site can't save a signed copy of a file with a password or editing restrictions, so if the checker reports encryption, ask the sender for an unrestricted copy before you place your signature.

Sources

• Adobe — Digital Signatures in a PDF (byte ranges, incremental updates, DocMDP permissions)
• Adobe Help — Permissions and limitations of signed PDFs
• Adobe Help — Certify PDFs
• Adobe Help — Validate digital signatures
• Adobe — Acrobat Sign security overview (certification of final signed PDFs)
• Docusign — Are electronic signatures safe? (digital sealing of completed documents)