Two passwords, wearing the same word
The PDF specification defines two separate passwords, and they do completely different jobs. Nearly every guide calls both of them “password protection”, which is how people end up trusting the weak one.
The open password, sometimes called the user password, is required before the document can be displayed at all. This is genuine cryptography. With modern settings the contents are encrypted with AES-256 using a key derived from your password. Without the password, a reader has nothing but encrypted streams. There is no clever trick, no recovery service and no workaround. The document is mathematically unavailable.
The permissions password, sometimes called the owner password, is a completely different animal. It does not hide the content. It sets flags in the file saying things like “printing is not allowed” or “text may not be copied”, and then it relies on the software opening the file to respect those flags voluntarily.
Why permission flags are not security
Nothing enforces them. A PDF with printing disabled is not encrypted against printing; it is a normal document carrying a note that asks readers not to print it. Well-behaved software honours the request. Plenty of other software does not, and stripping the flags entirely takes one command in tooling that has existed for twenty years.
This is worth being blunt about, because the flags are frequently used for exactly the situations where they fail. Sending a confidential report with copying disabled feels like protection. It is not. Anyone who wants the text will have it in under a minute, and they do not need to be sophisticated to manage it.
Permission flags have one legitimate use: signalling intent to people who are already inclined to cooperate. They tell a colleague that a document is not meant to be redistributed. Treat them as a polite notice on a door, not a lock.
What AES-256 actually buys you
When you set an open password, the document's content streams are encrypted. Your password is not used as the key directly, because passwords make poor keys. It is run through a derivation function that produces a proper 256-bit key, and that key encrypts the file.
AES-256 has no practical attack. Nobody is going to break the cipher. What
people do attack is the password, by guessing, and that is why the
strength of your password is the entire security of the document. A PDF locked
with 1234 is not protected by AES-256 in any meaningful sense,
because the attacker never touches the cipher. They guess the password in a
fraction of a second and the cipher politely opens the door.
Choose a password long enough that guessing is hopeless, and store it somewhere you will not lose it, because there is no recovery path. That is not a missing feature. A recovery path is precisely what encryption is designed to prevent.
AES-256 or AES-128
AES-256 is the default and the right answer nearly always. Every current PDF reader supports it, and there is no practical downside.
The single reason to choose AES-128 is an old reader. Acrobat 8 and earlier, and some embedded viewers in older hardware and kiosk systems, will simply refuse to open an AES-256 file. If a document is going to an organisation running software from before roughly 2009, AES-128 is still strong and it will actually open. Anything weaker, in particular the old 40-bit and 128-bit RC4 modes some tools still offer, should be avoided; those are genuinely broken.
Our PDF encryption tool offers both and defaults to AES-256.
The mistakes that undo the encryption
Sending the password with the file. A protected PDF and its password in the same email is one email's worth of security, which is none. Send the password through a different channel entirely.
Forgetting about the original. Encryption protects the file you made. The unprotected original in your downloads folder, in your sent items, or in a chat thread, is still unprotected. Encrypting a copy secures nothing if the plain copy is still lying around.
Assuming it hides who you are. An encrypted PDF still has a filename, a file size and a modification date, all visible to anyone who sees it. Encryption hides contents, not existence.
Trusting an upload. Most online PDF protection tools send your file to a server. You are handing an unencrypted confidential document to a third party, along with the password, in order to make it confidential. If the document is sensitive enough to encrypt, it is sensitive enough not to upload. Tools that run in your browser avoid the problem entirely, and you can verify the claim by disconnecting from the network and watching them still work.
A short decision guide
- Must not be read by the wrong person. Set an open password, use AES-256, choose a long password, send it separately.
- Would prefer it was not forwarded or printed. Permission flags communicate that, and nothing more. Do not mistake it for control.
- Both. Set an open password and a separate owner password. Readers need the first to open the document and the second to change its permissions.