AES encryption

AES (Advanced Encryption Standard) is the block cipher that secures a password-protected PDF. When you lock a document, the page content streams and strings are encrypted with AES, and the only way back to the readable bytes is to supply the right password and derive the correct key. Without it, the file on disk is just ciphertext.

PDF has used AES since version 1.6, first with 128-bit keys and, from PDF 2.0, with AES-256. The standard distinguishes two passwords: a user password that is required to open and view the document at all, and an owner password that governs permissions such as printing, copying or editing. It is worth knowing that permission flags depend on the reader honouring them, whereas the user-password encryption is the part that actually keeps content unreadable.

Because encrypting and decrypting requires handling the very content you are trying to protect, doing it locally is the consistent choice. reader.me applies and removes AES protection inside your browser, so neither the password nor the unprotected document is ever transmitted to a server that could log either one.