What a PDF can and cannot stop someone doing

PDF offers several features that look like security. They are not equivalent, and the difference between them is the difference between an obstacle and a lock.

Permissions flags: a request

A PDF can carry flags saying it may not be printed, copied from, or modified. These are requests. They are recorded in the document, and a well-behaved reader honours them by greying out the relevant menu items.

Nothing enforces them. A reader that chooses to ignore the flags does so trivially, and several do. Permissions are a politeness convention, useful for preventing accidents and useless against intent.

Owner passwords: a request with a lock on the wrong door

An owner password restricts what can be done with a document without restricting who can open it. The file is encrypted, but the key needed to read it is in the file — because the document has to open without a password.

This is genuinely weaker than it sounds, and tools that remove owner passwords are neither rare nor sophisticated.

User passwords: an actual lock

A user password is required to open the document at all. The content is encrypted with a key derived from it, and without the password there is nothing to read.

This is real protection, with real consequences. Lose the password and the document is gone. And it protects the file, not the information: once someone legitimately opens it, they have it, and can save an unprotected copy.

Watermarks: a signal

A watermark marks a document as draft, confidential or a copy. It communicates intent to a person looking at the page.

It stops nothing. Any PDF editor removes it, and on a text-based document it is simply another drawn object among many. Its value is social — a page stamped CONFIDENTIAL that turns up somewhere it should not is unambiguous about what happened. That is not nothing, but it is not a control.

Flattening: a one-way door for form data

Flattening a filled form draws the values into the page and removes the fields. Afterwards the answers cannot be edited in a reader and cannot be read back as structured data.

This is the one item on this list that reliably does what it claims — because it is not a restriction, it is a change to the document. That is the pattern worth noticing.

The rule underneath all of it

Restrictions on a document are requests. Changes to a document are facts.

Permissions ask a reader not to print. Flattening makes the fields not exist. Watermarking asks a person to treat a page carefully. Redaction makes the content not be there.

Anything in the first category can be undone by whoever holds the file. Anything in the second cannot, because there is nothing left to undo.

When you need a guarantee rather than a signal, ask which category you are in. If the answer is that a well-behaved reader will honour it, you have a convention. If the answer is that the document no longer contains the thing, you have a guarantee.