Two passwords that do completely different things
PDF encryption defines two separate passwords, and confusing them is the most common mistake in document protection. The user password — sometimes called the open password — is required to open the document at all. Without it the content is genuinely encrypted and unreadable. The owner password — the permissions password — leaves the document openable by anyone and merely records restrictions on printing, copying and editing.
The critical point is that owner-password restrictions are advisory. The file is technically encrypted, but the decryption key is derived in a way any conforming reader can compute without the password, because the reader must be able to display the document. Honouring a "no printing" flag is a politeness that compliant software chooses to observe, and free tools that strip these restrictions in seconds are trivially available.
So if the requirement is "only certain people may read this", set a user password and choose a strong one. If the requirement is "discourage casual copy-paste", an owner password does that and nothing more. Do not use it to protect anything whose disclosure would actually matter.
Encryption strength, and what it does not cover
The algorithm depends on the PDF version. Older files use RC4 at 40 or 128 bits, which is obsolete — 40-bit RC4 is breakable by brute force on ordinary hardware. PDF 1.6 introduced AES-128 and PDF 2.0 standardised AES-256, which is the only one worth choosing today.
AES-256 itself is not the weak point; your password is. The key is derived from the password, so a document encrypted with AES-256 and the password "summer2026" is protected by roughly the strength of "summer2026". Offline attacks against a PDF are unlimited — there is no server to rate-limit guesses, and an attacker holding the file can test billions of candidates. A long random passphrase from the password generator is the whole defence.
Note also what encryption does not hide. Document metadata may remain readable depending on the encryption mode, and the file's structure — page count, approximate size — is often inferable. Encryption protects content, not the fact that a document exists.
Delivering the password separately
Encryption is defeated entirely by sending the password in the same email as the file, which is nonetheless what happens most of the time. If the channel is compromised, both halves are compromised together, and you have achieved nothing beyond inconvenience.
Send the document one way and the password another — a message to a phone, a spoken call, a password manager's sharing feature. Avoid passwords derived from information the recipient's organisation publishes, such as a date of birth or an invoice number, since those are exactly what an attacker who intercepted the file would try first.
Finally, remember there is no recovery. A PDF encrypted with a strong user password and no record of that password is permanently unreadable — there is no reset link and no support desk. Store it in a password manager before you send the file, not afterwards.