How to sign a PDF that's already signed

First, find out how they signed

"Already signed" can mean four quite different files, and a method that's perfect for one breaks another:

A picture signature — drawn with a finger, typed in a script font, or scanned — that has become part of the page. This is the most common case by far.

A signature collected through an e-signature service such as DocuSign or Adobe Acrobat Sign. The name on the page may be typed or drawn, but the service seals the completed file with a certificate.

A certificate-based digital signature the other person applied themselves with a digital ID, for example in Adobe Acrobat.

A certified document: the author sealed it and set a permission level for anything added later.

The free checker on this site tells you which one you have in a few seconds, on your device. By eye: open the file in Adobe Acrobat Reader and look at the Signatures panel. Each digital signature is listed with its signer, and a valid certification shows a ribbon icon. If it lists nothing, the signature is a picture.

Case 1: a picture signature — sign it like any PDF

Nothing protects a picture signature except the page itself, so you can add yours with any PDF signer, including the one on this site: open the file, draw or type your signature, place it in your own block, add the date and download.

The other person's signature stays pixel for pixel where it was. What you must not do is change anything else — a typo fix or an updated figure in a document they've signed means they signed something different. Ask them for a corrected version instead.

Case 2: DocuSign, Acrobat Sign and similar services

Docusign says completed documents are digitally sealed with PKI, and Adobe says Acrobat Sign certifies final signed PDFs with a digital signature before sending them out. Downloading that PDF, adding your signature in another app and saving it produces a copy whose seal no longer validates.

The clean route is to sign inside the same service: ask the sender to add you as a signer, so your signature goes into the same sealed record. If their process has already finished, they may need to send a fresh request.

If that isn't possible and you both agree that a normal PDF signature is enough, sign a copy, keep their sealed original unchanged, and say in your reply that you've countersigned a copy.

Case 3: signed with a digital ID

The PDF format was designed for this. A later signer adds a new signature field and signs it, and the software appends the change to the end of the file instead of rewriting it, so the earlier signature keeps validating — unless the permissions set by the first signer or the author forbid it.

In Adobe Acrobat the path is All tools › Use a certificate › Digitally sign: drag a rectangle where your signature should go, pick your digital ID (if you have none, Acrobat offers to set one up under Configure New Digital ID), and save. If the first signer ticked "Lock document after signing", no further signatures can be added and you'll need a fresh copy from them.

If you don't have or want a digital ID, the other options are the same as in case 2: sign a separate counterpart, or agree that a picture signature on a copy is fine.

Case 4: a certified document

Certification comes with a permission level chosen by the author: no changes at all; form filling and signing; or form filling, signing and comments. With "no changes", anything you add breaks the certification. With the other two, you can sign an existing signature field with a digital ID, but placing a picture signature on the page and saving a new file still counts as a forbidden change.

If there's no field for you, or no changes are allowed, ask the author for a version prepared for your signature. Adobe's help pages point the same way: once other people have signed, the fix is an unsigned copy from the document's owner, or the signer clearing their own signature.

When the app says “This document has been signed and cannot be edited”

That message means the lock is doing its job: protecting what someone signed. According to Adobe, signing a PDF with a digital ID locks it against further changes, and the file becomes read-only for others. The signature covers the exact bytes of the document, so the app stops you from editing content that would invalidate it. Adding further signatures usually still works (see cases 3 and 4 for the exceptions); editing the text doesn't.

You're the only signer and need to change something: Adobe's advice is to right-click your signature and choose Clear Signature, make the change, then sign again — unless you locked the document, in which case you'll need your unsigned original.

Others have signed and something must change: ask them. Adobe's guidance is to request the original unsigned copy, or to ask the signer to remove their signature. A change after signing is a new agreement and needs their consent anyway.

The Signatures panel is empty but editing is still refused: the PDF probably isn't signed at all but carries a permissions password — look at the Security tab of the document properties. The message looks similar, but the fix is different: ask the sender for the password or an unrestricted copy. Don't run someone else's document through a password remover.

Why editing it anyway is a bad idea

Plenty of tools will open a locked PDF and let you type over it, and "printing" it to a new PDF produces a copy without any signature protection. Technically that works. But the new file no longer proves anything: the other party's digital signature won't validate in it, and if you changed content they had signed, it's a document they never agreed to.

A flattened copy is fine as your own reading or note-taking copy. It is not a substitute for the signed original, so never send it back as though it were the agreed contract.

Adding text, a date or initials after someone signed

The same rules apply to anything you add, not only signatures. On a picture-signed PDF you can type your name, title and date beside your signature freely. On a digitally signed or certified one, filling in existing form fields may be allowed, but typing text onto the page and saving a new file isn't — it changes the signed content.

If you're the one sending documents: avoid the lock

Adobe recommends saving an unsigned backup before signing and not ticking "Lock document after signing" when more signatures are needed. Two more habits help: if you certify, allow form filling and signing; and add a signature field for every signer before the first person signs.

Or keep it simple. For ordinary agreements between two parties, picture signatures on a plain PDF — first signer, then countersigner — avoid these locks altogether.

Sources

• Adobe — Digital Signatures in a PDF (multiple signatures, incremental updates, DocMDP)
• Adobe Help — Add digital signatures to PDFs in Acrobat
• Adobe Help — Permissions and limitations of signed PDFs
• Adobe — Acrobat Sign security overview
• Adobe Help — Certify PDFs
• Docusign — Are electronic signatures safe?