Passwords in Adobe Acrobat
Standard PDF security
Protected PDF document has two passwords: an owner password and a user password. The document also specifies operations that should be restricted even when the document is decrypted: printing; copying text and graphics out of the document; modifying the document; and adding or modifying text notes and AcroForm fields.
Applications for password recovery:
Advanced PDF Password Recovery
http://www.elcomsoft.com
Password types
When the correct user password is supplied, the document is opened and decrypted but these operations are restricted; when the owner password is supplied, all operations are allowed. The owner password is required to change these passwords and restrictions.
Encryption key
Protected PDF document is encrypted with the RC4 algorithm. Encryption key length is 40 bits. Key is calculated from the user password. Knowing of the owner password allows calculation of the user password and therefore encryption key. All restrictions are enforced by software, not by PDF format itself.