Free · iPhone, iPad and Mac
Sign a signed PDF without breaking the first signature.
A contract arrives already signed by the other side. You fill your part, sign, and send it back. With most apps, the first signature quietly stops validating. With Keepsign it doesn’t, and you get a receipt that says so.
The signature covers bytes, not looks.
A digital signature is a fingerprint of the exact bytes in the file when it was signed. Change one byte inside that range and the fingerprint no longer matches. The page can look identical and still fail.
Rewrite everything
Many apps rebuild the whole PDF when you save. Every byte moves, so the earlier signature can’t match any more, even if you only typed your name.
Add at the end
The PDF format lets you append changes after the signed part, called an incremental update. The signature keeps covering what was signed, and a validator sees what came later.
Add, check, then report
Keepsign always appends. Then it reopens the file, verifies each signature again and tells you the result. If a change can’t be appended safely, it asks first.
What you can do without breaking anything.
Fill the fields left for you
Text, checkboxes, lists, dates. Calculated fields update without running the file’s scripts.
Add your signature or initials
Draw it with a finger, Apple Pencil or trackpad, type it, or take it from a photo. It stays as a sharp vector line, not a blurry picture.
Add notes and highlights
Annotations are allowed changes for most signed documents, and Keepsign checks that the one you have allows them.
Sign with a Digital ID, too
Add your own certificate signature after theirs. Each signature covers its own revision. More on certificate signatures.
Save and read the receipt
“Your signature stays valid”, with each signature before and after. Attach the receipt when you send the file back if you like.
Some changes break a signature by nature.
Rewriting text on a signed page, deleting a page, or flattening a field that was signed is a change the signer didn’t approve. No app can make that pass.
Keepsign won’t pretend otherwise. Before saving, it shows which signature would stop validating, whose it is and why. You can cancel, save a separate copy, or go ahead knowing the result. Organizations can turn “save anyway” off entirely.
▲Ana Ruiz signed this document
Editing the text on page 2 changes what she signed. Her signature would show as not valid.
●Filling fields and adding notes are fine
Those are permitted after her signature and keep it valid.
Questions
Why does my PDF signature become invalid after I edit or fill it?
Because the app saved by rewriting the file. The signature still sits there, but the bytes it protects have moved or changed. Our article on why PDF signatures show as invalid goes through every common cause.
Can I add my signature after someone else signed?
Yes, as long as the document allows more signatures. Each signature is appended as its own revision, so the first one keeps covering what it covered.
Is signing free?
Yes, always: drawn, typed or photo signatures, initials, dates and Digital ID signatures, one document at a time. One saved signature and one set of initials are free, and they sync through iCloud if you like. With Pro you get as many saved signatures as you need, each with its name. Signing itself is never limited: any new signature can be used just this once.
How will the other person’s software see my changes?
Signature validators, Adobe Acrobat among them, separate changes the signer permitted, such as filling fields, from other changes made after signing. Keepsign’s receipt uses the same distinction, so you know in advance which side your change falls on.
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