A PDF file may be encrypted for security, or digitally signed for authentication.
The standard security provided by Acrobat PDF consists of two different methods and two different passwords, "user password", which encrypts the file and prevents opening, and "owner password", which specifies operations that should be restricted even when the document is decrypted, which can include: printing, copying text and graphics out of the document, modifying the document, or adding or modifying text notes and AcroForm fields. The "user password" (controls opening) encrypts the file and requires password cracking to defeat, with difficulty depending on password strength and encryption method – it is potentially very secure (assuming good password and encryption method without known attacks). The "owner password" (controls operations) does not encrypt the file, and instead relies on client software to respect these restrictions, and is not secure.
References:
Portable Document Format Security:
http://en.wikipedia.org/wiki/Portable_Document_Format#Security_and_signatures