Will compressing a signed PDF break the signature?

It depends on which kind of signature the PDF has. A picture of a signature, drawn in Preview or scanned from paper, is just an image on the page and survives compression like any other image. A certificate-based digital signature is a cryptographic seal over the exact bytes of the file, and compressing, re-saving or flattening the PDF changes those bytes, so the signature stops validating or disappears altogether. The fix is to compress before signing, or to ask for a smaller signed original.

Two very different things called a signature

A picture of a signature. Your name drawn on the trackpad with the signature button in Preview’s Markup toolbar, a scan of ink on paper, an image pasted into a Word document before exporting it. It looks like a signature and that is all it is. Preview places its signatures on the page as annotations, much like a stamp. Anyone could copy one into another document, and nothing in the file proves who added it or that the page has not changed since.

A certificate-based digital signature. Made with a digital ID: a certificate issued to a person or an organization, held on a smart card, in a keychain, or by a signing service. The PDF contains a cryptographic signature calculated over the file, plus the certificate of whoever signed. A reader that checks signatures can confirm who signed and that the signed bytes have not changed since. Documents issued through government and business portals often carry one, and many e-signature services seal the finished document this way even when you signed by typing your name.

A digital signature may or may not show anything on the page. A box reading “Digitally signed by” followed by a name and a date is a hint, not proof, because a picture can look exactly the same.

Why compressing breaks a digital signature

A certificate signature records precisely which bytes of the file it covers, as a /ByteRange entry: normally everything from the start of the file up to the signature, and everything after it to the end. The gap between is where the signature itself is stored. When a reader validates, it fingerprints those byte ranges and compares the result with the signature. Change a single byte inside them and the comparison fails.

Compression rewrites the whole file: images are re-encoded, objects are reordered, the structure is rebuilt. So does re-saving in most PDF apps, printing to PDF, flattening form fields or annotations into the page, and rotating or deleting a page.

In one test we made two copies of a signed PDF. The copy re-saved through the PDF framework built into macOS, the one Preview uses, came out with no signature in it at all. The copy run through a command-line tool that only reorganized and recompressed the file’s structure, without touching a single image, kept the signature data, but the bytes underneath had moved, so it no longer matched. Neither copy would pass a validity check.

Picture signatures are a different matter. They are images, and compression treats them like any other image. A heavy setting can make a fine pen line fuzzy, so open the compressed copy, zoom to 100 percent, and make sure the signature is still crisp.

How to tell which kind you have on a Mac

1. Think about how it was signed. Drawing, typing or pasting a signature yourself produces a picture, although an e-signature service may add a certificate seal when the document is finished. Being asked for a digital ID, a smart card, a PIN or a certificate means a digital signature.
2. Open it in a PDF reader that validates signatures. It shows a signature panel or a banner stating whether each signature is valid. Preview displays the page, including any visible signature box, but it does not tell you whether a certificate signature still holds.
3. Ask Terminal. A certificate signature in a PDF carries a /ByteRange entry, the list of covered bytes described above. Open Terminal, type the command below up to and including the closing quote and a space, then drag the PDF onto the window to fill in its path (your-file.pdf below is a placeholder):

grep -a -c "/ByteRange" your-file.pdf

A result of 0 means there is no certificate signature in the file. Any number above 0 means there is at least one. To see what kind:

grep -a -o "/SubFilter */[A-Za-z0-9.]*" your-file.pdf

The answer is a standard name for the signature format. Two common examples are /SubFilter /adbe.pkcs7.detached and /SubFilter /ETSI.CAdES.detached; both mean a certificate signature. Neither command changes the file.

What to do when a signed PDF is too big to send

1. Send it unchanged, by another route. A shared link from a cloud folder, or an upload portal with a higher limit, keeps the signature intact.
2. Ask for a smaller original. If someone else signed it, ask them to compress the document before signing and send it again. Only the signer can produce a new valid signature.
3. If you are the one signing, compress first. Shrink the unsigned PDF to the size you need, check it reads well, then sign the shrunk copy. The order is the whole trick.
4. If the recipient only needs to read it, a compressed copy may be acceptable, but say that it is a compressed copy and keep the signed original. Do not assume; some recipients check.

For a PDF signed with a picture, none of this applies, and you can compress it the normal way. How to compress a PDF on a Mac covers Preview’s Reduce File Size filter and gentler options.

Where Manatee fits

Manatee has a Shrink mode: drop in a PDF, type a size in MB, and it finds the quality that fits. It always writes a new copy beside the original and never modifies the original, so running it on a signed PDF leaves the signed file exactly as it was. The shrunk copy, like the output of any compressor, will not carry a valid certificate signature.

That makes it useful in two of the cases above: shrinking a document before you sign it, and getting a picture-signed PDF under an upload limit. Everything runs on the Mac with no upload and no account, which is reasonable to want for a contract, and the first 24 hours are free with every feature.

Questions

I already compressed it and the signature shows as invalid. Can it be fixed? Not by any tool. Making the signature valid again would need the signer’s private key, which by design only the signer has. Go back to the signed original, or ask the sender for it again. A PDF that broke after you saved changes to it in Preview covers other damage a re-save can cause.

Can I add my own signature after someone else has signed? Often, if your signing app adds the new signature as an incremental update, appending it to the end of the file and leaving the earlier signed bytes alone. An app that rewrites the whole file when it saves breaks the first signature. Some documents are also locked by their first signer against later changes. Try it on a copy before signing the real thing.

Does a scanned contract with ink signatures count as digitally signed? No. A scan is a picture of paper, so compressing it is safe as long as the text and signatures stay legible. Check the page at 100 percent before you send it.