How to Reduce the Size of a Digitally Signed PDF

This is a genuine conflict rather than a technical obstacle. A digital signature covers the bytes of the file, so any change to those bytes breaks it. Compression changes the bytes. You cannot compress a signed PDF and keep the signature valid — and any tool claiming otherwise is either not really compressing it or not really preserving the signature.

So there are only three honest options.

1. Compress first, then sign

The right answer whenever you control the order. Optimise the document, then apply the signature to the final file. The signature then covers what you actually intend to distribute, which is also better practice — signing before finalising means signing something that was going to change.

2. Ask the signer to re-sign a compressed copy

If the document is already signed and must stay small and verifiable, the only route is: strip the signature, compress, and have the original signer sign again. That requires their cooperation and their certificate; there is no way to do it on your side alone. That is precisely what signatures are for.

3. Accept the size, or send it differently

Often the pragmatic answer. If the signature must remain valid, keep the file intact and solve the transfer problem instead — a shared link rather than an email attachment. Most "this file is too big" problems are really "my mail server rejects it" problems, and a link sidesteps them without touching the document.

If the signature does not actually need to stay valid

Many documents are signed for appearance rather than verification — a visible signature image on an agreement, where nobody will check the certificate. If that is your case, you can flatten the signature into the page and compress freely. Compress PDF accepts up to 100 MB.

Be clear-eyed about the trade: flattening converts the signature into a picture of a signature. It loses cryptographic meaning entirely. Fine for an internal agreement, wrong for anything where the signature might be challenged.

Why signed PDFs are large in the first place

Usually not the signature — which is small — but what was signed. Scanned pages, embedded fonts, and often an incremental save history, because signing appends rather than rewriting. A document signed several times carries every previous version inside it.

Frequently asked questions

Can I compress a signed PDF without breaking the signature? No. The signature covers the file's bytes; compression changes them. Anything claiming otherwise is misrepresenting one side of it.

Will the signature still look right after flattening? It will look identical and mean nothing cryptographically. The visible mark remains; the verification does not.

Why did my file grow after signing? Incremental save. Signing appends a new revision and keeps the old one.

What if I signed it myself? Then you can strip, compress and re-sign with your own certificate — the straightforward case.