Preview can’t sign with a certificate — and it breaks signatures

Preview is a fine PDF reader, and opening a signed document in it does no harm. Two things it can’t do are worth knowing before you trust it with a contract: it can’t make a digital signature, and saving a PDF that already has one breaks it. You don’t even have to change anything. Opening and saving is enough.

What Preview’s “Sign” actually does

Apple’s guide to signing in Preview describes three ways to create a signature — the trackpad, the camera, or an iPhone or iPad — and then placing it on the page. It doesn’t mention certificates or digital IDs anywhere.

What you get is a visual signature: an image of your handwriting. In the US that can be a perfectly good electronic signature. But it isn’t a digital signature. There’s no certificate behind it, and nothing in the file would show if someone moved it or changed the text around it. The difference is spelled out in a visual signature is not a digital signature.

What we measured

On October 14, 2026, on a Mac running macOS 27.0 (build 26A428), we took a PDF signed in PAdES B-LT that our validator reported as valid and unaltered, and made two copies.

• The first we opened in Preview and closed without saving. The file stayed identical, byte for byte.
• The second we opened, saved without changing anything, and closed. Preview wrote the whole document out again, and the original bytes were no longer at the start of the file, which is where the signature expects them.

In the second copy the signature was still there, with its certificate and its cryptography intact. But what it covered was no longer what the file contained. The verdict went from valid to invalid, with the reason “the document was modified after it was signed”. The metadata gained a new producer naming macOS 27.0, build 26A428, and “Quartz PDFDocument”.

We repeated the test with a document certified with no changes allowed, which is how its author says nobody should touch it. Preview saved it all the same. Saving through PDFKit, Apple’s framework for reading and writing PDFs, gave the same result: the whole file rewritten and the signature invalid. We ran the PDFKit test a second time, separately, the same day, on another signed B-LT test file: same outcome.

Why the signature stops being valid

A digital signature in a PDF covers specific bytes of the file: all of them except the gap where the signature itself sits. To add something to a signed document without breaking it, the new material is written after the old, as one more revision, and what was signed stays exactly where it was. That’s how a second signature, a timestamp or a careful annotation gets added.

Rewriting the whole file does the opposite: it rebuilds and rearranges those bytes. What you see on screen can look identical, but the validator’s arithmetic no longer adds up, and the signature can no longer prove that this is the document that was signed.

It depends on the macOS version

This isn’t new. In January 2023, on macOS Ventura 13.2, a user on Adobe’s community forum reported being able to delete, from Preview, a signature made in Acrobat. A community expert explained that Preview ignored the document’s permissions and appended its save to the end of the file, so the PDF appeared unsigned.

On macOS 27 the mechanism is different — nothing is appended, the whole file is rewritten — but the outcome is the same: the signature doesn’t survive the save. We haven’t measured other macOS versions. If you use one, check as described below before trusting a document that has been through Preview.

How to tell if it happened to you

Validate the PDF. That’s the check that counts. If a document that reached you signed now shows as modified or invalid, something touched it afterwards. How to check whether a PDF signature is valid walks through three free ways to do it.

Look at the producer. A PDF’s document properties, in Acrobat Reader for example, show which program wrote it last. If it names macOS and Quartz, the file has been saved by Apple’s PDF engine. If it doesn’t, that proves nothing: other apps rewrite PDFs too, and each leaves its own name, or none.

Compare with the original. If it came by email, the attachment is still there. Validate that too: if the attachment passes and your copy doesn’t, you know where it broke.

macOS keeps earlier versions of documents in many apps, and Apple explains how to get them back with File > Revert To > Browse All Versions (Apple’s help). We haven’t tried it with a signed PDF, so don’t count on it.

How to avoid it

• Don’t save a signed PDF from Preview. Reading it does nothing; saving it does.
• Be careful with annotations and form filling. Apple’s same help notes that many apps save versions automatically as you work, so a quick note can end up saved without you pressing Save. If you must annotate a signed document, do it on a duplicate and keep the original.
• To add anything to a signed PDF, use an app that writes incremental revisions. PDFsigna always does, into a new copy, without touching the original. If the document is already signed, it warns you first and validates the signature again after saving, as described in how it works. The Windows version is on the Microsoft Store; the Mac version is in Apple’s review.
• Always keep the original you were sent. It’s the proof of what was signed, and it costs nothing.

And if what you need is to sign with a certificate on your Mac, start with which certificate you need.

This article was written with the help of artificial intelligence.

This article is general information, not professional advice. Laws, prices and how software works change over time: before you decide, check the official, up-to-date documentation from each authority or vendor.