How to add your signature to a PDF someone else already signed on iPhone

Someone signed a PDF and sent it to you. Now it’s your turn, and you’re hesitating over the same thing everyone hesitates over: can you add your signature without breaking theirs?

Almost every page you’ll find on this searches answers the generic question — how to sign a PDF on an iPhone — because that’s the page the big signing companies built. It skips the part you actually care about. So here it is first.

The short version

In the ordinary case, yes, and their signature is in no danger. Most signatures on PDFs people email around are images that have been printed into the page. They’re pixels, no different from the document’s own text. You can draw, type and place things on top of them all day; the earlier signature can’t be moved, damaged or deleted by anything you add.

There is exactly one case where it’s different, and it’s the reason this page exists: a PDF carrying a certificate-based digital signature. That’s a cryptographic seal over the file’s exact bytes, and any edit — yours, Markup’s, ours, anyone’s — changes those bytes and makes the seal report as broken. If that’s what you’ve got, the file will usually refuse to let you edit it in the first place, and the fix is not a workaround. It’s a conversation with the sender.

Ninety seconds of checking tells you which one you’re holding.

First: which kind of “already signed” is this?

The common kind — a flattened image signature

This is what you get when the first person signed with iOS Markup, with an app like Signed, by printing and scanning, or by dropping a signature PNG into Word before exporting. The signature is a picture that has been flattened into the page. It’s part of the artwork now.

Nothing you do on a different part of the page touches it. This is the overwhelming majority of “already signed” PDFs.

The other kind — a certificate signature

This one comes from Adobe Acrobat’s Sign with a Digital ID, a completed DocuSign or Dropbox Sign envelope, or a government-issued signing card. Behind the scenes it stores a hash of the file plus a certificate identifying the signer. Change one byte and the hash no longer matches, so any viewer that checks it will say the signature is invalid or the document has been modified since signing.

Documents like this are often marked signed and locked by whoever signed them, which is why they open read-only.

The check, on your iPhone

1. Open the PDF in Files.
2. Tap the Markup (pen tip) icon.
3. Try to add anything — a scribble, a text box.

If you can add and move things freely, it’s the ordinary kind. Carry on to the next section.

If Markup is greyed out, refuses to save, or the file opens read-only, treat it as certificate-signed and skip ahead to what to do when the file is locked. One more clue worth knowing: open the same file in the free Adobe Acrobat Reader app and a certificate signature shows up as a banner across the top naming the signer. An image signature shows nothing, because there’s nothing to report.

If it simply asks for a password, that’s a third thing entirely and it’s easily solved — see signing a password-protected PDF on iPhone.

The free route: iOS Markup, on the file they sent you

No app, no account, no cost. This is the honest first answer and for a one-off it’s genuinely enough.

1. Open the PDF from the message they sent — tap the attachment in Mail, or save it to Files first. Do not go and find your own blank copy of the document. More on why in a moment.
2. Tap the Markup icon.
3. Tap + → Signature. If you’ve drawn one on this iPhone before it’s already in the list; otherwise tap Add or Remove Signature → + and draw yours with a finger. Draw bigger than feels right — it scales down cleanly.
4. Place it on your line. Drag the corner handles to size it. Take a second to check you haven’t parked it over the other person’s signature or over any text.
5. Dates and initials, if the document asks for them, are separate text boxes: + → Text, type, drag into place.
6. Tap Done, then Share the file back.

The honest limitations, because they’re the reason people go looking for something else:

• Your signature stays an annotation until something flattens it. In that state a recipient can drag it, delete it, or watch it fail to print — the exact problems described in flattening a signed PDF. Their flattened signature is fine; yours is the fragile one. Print → Share → Save to Files fixes it in about fifteen seconds.
• Dates and initials are manual boxes you nudge into position, one at a time. On a document that wants your initials on every page, this gets old fast — see adding initials to every page of a PDF.
• Finger-drawing on glass usually takes a few goes.

The mistake that produces two half-signed files

Worth naming on its own, because it’s the most common way this goes wrong and it’s invisible until someone asks for the executed copy.

Sign the file they sent you. Not your own copy of the original.

If you dig out the blank version from the earlier email and sign that, you now have two documents — one with their signature, one with yours — and no single file carrying both. Whoever signs second is the one who has to get this right, and on this document, that’s you.

The mirror image of this page, for when you’re the one starting the chain, is getting two people to sign the same PDF on iPhone — that one covers the sender’s side: finishing the document before anyone signs, and flattening your half before you pass it on.

Doing it in Signed

If you sign things more than occasionally, the tedious parts above are worth removing. Here’s what’s different, described accurately.

You import the PDF they sent — straight from the email attachment via the share sheet. Signed opens the document exactly as it is, their signature and all, and lets you place your own fields on top: your saved signature, your initials, today’s date, any text the form needs. Tap Finish and it writes a new file with your fields drawn into the pages.

Two things follow from how that works, and both matter here:

• It draws on top; it never edits what’s already on the page. The first signature is existing page content, and Signed adds to pages rather than rewriting them. There is no step where their signature could be moved or removed.
• The copy you get back is already flattened. No annotation layer, so no Print-to-PDF detour and no version of the file where your signature is a draggable sticker. Which matters more than usual when you’re the second signer — you’re handing this to someone who’s about to file it.

Your signature is saved once and reused, so the second document takes seconds, and the whole thing runs on the device — Signed doesn’t upload your documents, which is worth something when the file already has someone else’s signature on it as well as yours.

And the boundary, stated plainly: Signed is not a certificate tool. It doesn’t create certificate-based signatures, it doesn’t validate them, and it can’t preserve one that’s already on a file. Like every editor, it re-saves the document. If the PDF you were sent carries a certificate signature, Signed is the wrong tool and so is Markup — read the next section instead.

When the file is locked, or certificate-signed

You’ve found that you can’t edit it. Don’t go hunting for a way around the lock; the lock is doing its job, and stripping it produces a file whose seal now reports as broken — which looks considerably worse to the recipient than an unsigned one.

What actually works:

• Ask the sender for a copy you can sign. Usually the fastest fix and usually uncontroversial. In Acrobat, signed and locked is a checkbox, and plenty of people tick it without meaning to close the door on the second signer.
• Sign inside the platform they used. DocuSign, Dropbox Sign, Adobe Acrobat Sign and the rest are built for exactly this: multiple signers in sequence, each addition wrapped in its own signature rather than pasted over the previous one. If the document arrived from one of those, ask to be added as a signer. You won’t need a paid account to sign as a recipient.
• If a certificate signature is required of you too, that’s a certificate tool’s job, not an iPhone annotation’s. Nothing on this page produces one, and no page that tells you otherwise is being straight with you.

For the everyday case — a lease, a freelance agreement, a permission slip, an NDA between two people who trust each other — you will almost never meet this. It’s worth thirty seconds of checking precisely because the answer is nearly always “you’re fine, go ahead”.

Sending it back

Whatever route you took, three things before you hit send:

1. Open the finished file and look at it. Both signatures visible, on the right lines, nothing overlapping.
2. Check yours is flattened — open it in Markup and try to tap your signature. If it selects and drags, flatten it before sending.
3. Reply in the same thread with the file attached, and say plainly what it is: “Signed and attached — this is the copy you sent, with my signature added.” It saves the other person from wondering whether they’re looking at a second half-signed version.

One thing this page deliberately doesn’t answer: whether a particular recipient, institution or jurisdiction will accept a countersigned PDF made this way. That depends on the document and where you are, not on how you produced the file, and some documents want a witness or a notary no matter what. Are electronic signatures accepted? is the honest version of that question. Signed is a signing tool, not legal advice.

The short answer

Check whether the existing signature is an image or a certificate. If it’s an image — which it nearly always is — open the file they sent you and add your signature on top with Markup or Signed; theirs is baked into the page and cannot be disturbed. Flatten your own signature before sending it back. If the file is locked or carries a certificate signature, don’t try to edit it — ask the sender for a signable copy or sign inside the platform they used.