What happens to signatures when you merge PDFs
Checked against our tools on 10 October 2026
It depends on the kind of signature. A signature that's a picture on the page, like one drawn with Sign PDF, is part of the page and survives a merge unchanged. A certificate-based digital signature proves the file hasn't changed since signing, so merging, which writes a new file, makes it invalid. Merge first, then sign.
Go straight to the tool: Merge PDF, Sign PDF
Two kinds of signature
An image signature is a picture of a signature placed on a page. It might be drawn with a mouse or finger, typed in a script font or photographed from paper. Our Sign PDF tool makes this kind. To the PDF, it's just page content, like a logo.
A digital signature (sometimes called a certificate signature) is different. It's a cryptographic seal stored inside the file, made with a certificate that identifies the signer. A PDF reader checks it by recalculating a fingerprint of the file's exact bytes and comparing it with the one in the seal. If even one byte has changed, the check fails.
Why merging breaks digital signatures
Merging creates a new PDF and copies the pages into it. The new file has different bytes, so the fingerprint no longer matches, and the signature can't be valid any more. This isn't specific to our tool: any tool that writes a new file has the same effect, which is what digital signatures are designed to detect.
In our test, we merged a sample lease containing a signature field. The visible signature box was copied to the merged file, but the field was no longer part of the document's form and the file's bytes had changed, so there was nothing left that could validate. Expect a reader that checks signatures to treat the merged document as unsigned.
What survives, in short
| Signature type | After merging |
|---|---|
| Drawn, typed or photographed signature image | Unchanged, it's part of the page |
| Certificate-based digital signature | No longer valid |
| Signature fields waiting to be signed | No longer usable as fields |
How to combine signed documents
Pick the option that fits what the recipient needs:
- Merge first, then sign. Build the complete document, then send it for signature. This is the cleanest option.
- Send signed files separately. If each document was signed by someone else, keep the signed originals as they are and send them together, for example in one email or a ZIP.
- Sign again after merging. If the recipient needs valid signatures on the combined file, everyone who signed has to sign the merged version. No merge tool can carry a signature across, because it can't sign on the signer's behalf.
The same applies to other edits
Compressing, rotating, adding page numbers, watermarking or removing pages all write a new file too, so each of them invalidates a digital signature. Do these before signing. Image signatures aren't affected by any of them.