Adobe Acrobat features two levels of password protection.
Protecting documents with an access restriction password such as the "owner", "security" or "master" password does not affect the ability to open and view the PDF file, but prevents users from altering the file, printing it, selecting text and graphics and copying items to the Clipboard, adding or changing annotations and form fields and so on, in any combination. You can view the restrictions in Adobe software by using the File | Properties, Security, Show Details:
APDFPR can instantly remove these restrictions if the "user" password is either not set or is known.
In addition, there is also the "open" ("user") password. If one is set, the file will be encrypted with a strong algorithm. It cannot be opened if the password or encryption key is not known. APDFPR can attempt to recover this password via dictionary and brute-force attacks. In addition, APDFPR allows to attacks the "owner" password, because decrypting the file is possible with either the "user" or "owner" password. Even if both passwords are long and complex, PDF documents created in legacy versions of Adobe software can be decrypted using the Key search attack, which tries all possible 40-bit RC4 keys. It may take hours or even days to complete, but the decryption is guaranteed. Pre-computed hash tables shipped with APDFPR Enterprise reduce the wait to just a few minutes.
Note that once the file is saved in Adobe Acrobat, and the "user" password is set, the "owner" password is set automatically to the same value (but can be changed manually). A PDF file cannot have only the "user" password, so it always has either the "owner" password, or both "owner" and "user" passwords (which can be the same or different). Please take that in mind when selecting Advanced options.
Finally, PDF files can be protected using Digital Rights Management (DRM) or third-party plugins such as FileOpen. APDFPR does not support these protection methods.
Note that Acrobat versions 5 through 8 can create PDF files with an improved security level: 56..128-bit RC4 or 128-bit AES encryption. For these files, the "owner" protection can be recovered instantly as for Adobe Acrobat 4.0 (and older versions), but brute-force and dictionary attacks are available yet very slow. The "key search" attack is not available. For Acrobat 9 and newer files with 256-bit AES encryption, "key search" attack is not available, but the brute-force attack speed is significantly faster.
When the brute-force or dictionary attack starts, APDFPR provides additional information about the type of security handler used; the log window will contain a record e.g.
05.04.2002 13:05:51 - File "C:My Documents\test.pdf" opened.
05.04.2002 13:06:14 - Handler: Acrobat Standard (Standard) 40-bit security v.1.
or
05.04.2002 13:05:51 - Handler: Acrobat Standard (Standard) 128-bit security v.2.
PDF files (even unencrypted) may also contain additional objects such as JScript code, form fields and digital signatures; sometimes they are used for document protection. APDFPR allows to remove them as well.