Compress a signed PDF without breaking the signature

Which kind of signature do you have?

There are 2 things people call a “signed PDF”, and they behave in opposite ways.

	Drawn, typed or scanned	Certificate (digital) signature
What it is	Ink on the page: a drawing, outlined text, or part of a scanned picture	A cryptographic seal over the file’s bytes, plus a picture of it
Where you see it	On the page	On the page, in Acrobat Reader’s Signatures panel, and counted by getPDF’s inspector
After compression	Still there; a scanned one may soften a little	Still visible, but reported as invalid
Safe to compress	Yes	No

The quickest check is our inspector, which runs on your device: drop the file and read Digital signatures in the Security section. “None” means any signature you see is ink on the page. A number means the file carries certificate signatures, and the result says so in words: rewriting the file, compression included, breaks them, so keep the original. The inspector counts signatures; it does not check whether they are valid.

Inspect PDFFonts, images, metadata, why it is big. Free, runs on your device.Open

To see who signed and whether the seal still holds, open the file in Adobe Acrobat Reader, which is free. A certificate-signed file shows a message bar at the top about its signatures, and the signatures are listed in the Signatures panel (Adobe’s help names it as the panel on the right, or View, Show/Hide, Navigation Panes, Signatures; checked on 11 October 2026).

Do not use a browser tab to judge validity. A browser draws the signature’s appearance, and that picture looks the same whether the seal behind it is intact or broken.

Why compressing breaks a certificate signature

The PDF standard (ISO 32000) defines a certificate signature as a hash of the file’s bytes, sealed with the signer’s key. The signature records exactly which byte ranges it covers: the whole file except the slot where the signature value itself sits.

Compression works by rewriting the pictures inside the file, so millions of covered bytes change and everything after them moves. The reader recomputes the hash, it no longer matches, and Acrobat Reader reports the signature as invalid. The signature’s picture still sits on the page, which is what makes this trap easy to fall into: the file looks signed and is not.

The same is true of every tool that writes the file anew: splitting, extracting pages, flattening, and “print to PDF” all produce new bytes. Our flatten tool’s page states it plainly: flattening invalidates a digital signature, as any change does. getPDF’s compressor writes a new file too, so it breaks a certificate signature like any other. It does not stop you, but every getPDF tool that writes a PDF from a signed file says so on the result: the file is digitally signed, the new copy’s signature no longer checks out, and you should keep the original for anything that counts.

Could a tool keep the seal and still shrink the file? No. The standard allows changes to be appended after the signed bytes (that is how a second person signs), but the original bytes, pictures included, must stay in the file for the first signature to verify. Appending can only make the file bigger. getPDF’s editor saves a signed file exactly that way, so its signature stays valid for what was signed, and it tells you that the signed version is still inside the file.

Certificate-signed and too big: what to do

1. Send it as a link. Upload the signed file unchanged to the cloud storage you already use and share the link. The bytes arrive intact, so the signature still verifies. The guide to sending a large PDF covers who can open such a link and for how long.
2. Ask the signer for a smaller original. If the person who signed still has the unsigned document, they can compress it first and sign the smaller file. A signature applied after compression is valid.
3. Ask the receiver what they accept. Some portals take a file over their stated cap by email, or accept the signed original on paper. Ask before you break anything.

Do not split it into parts to fit an email cap: each part is a new file, and none of them carries a valid signature.

Drawn, typed or scanned signature: compress normally

If your signature is ink on the page, compress the file as you would any other:

1. Drop the file and pick Email, or type a size target.
2. Download the result and open it.
3. Zoom to 200 percent on the signature and check that it is still clear.

What happens to it depends on how it got there. A signature drawn or typed in our signing tool is stored as vector lines, and the compressor keeps drawings as they are, so it does not change at all. An uploaded photo of a signature is a picture, so it is re-encoded like any other picture. On a scanned paper contract, the signature is part of the page scan and is shrunk with it; if it looks too soft, rerun with the Web preset.

Next time: compress first, then sign

The order matters. Compress the document, check it, and only then apply a certificate signature, or ask the signer to. A signature added last covers the final, small file and stays valid wherever it travels.

For drawn signatures the order matters less, but the same habit saves a step: compress, then sign, then send. The sign and fill guide covers signing on your device, and PDF privacy and protection explains when a certificate signature is worth having at all.

What this page cannot tell you

We explain the mechanics, not the law. Whether a re-signed copy, a paper original or a link is acceptable is the receiver’s call, and sometimes a regulator’s; this is not legal advice. getPDF does not make or validate certificate signatures today; Adobe Acrobat Reader validates them for free. Everything getPDF does do runs on your device, so nothing is uploaded while you work out the right route.