Like word documents, .PDF files created by Adobe's Acrobat software can be protected by two different levels of passwords. A 'user' password prevents anyone from opening the file unless they provide the correct identification, while an 'owner' password allows reading but prevents various forms of modification unless the correct password is entered.

Both of these forms of protection are susceptible to dictionary and brute-force attacks, as are most other programs that offer password protection. PDF files protected by older versions of Acrobat are also vulnerable to key searches, in which each possible numerical key (generated from the password to encrypt and decrypt the document) is tried.

Newer versions include support for encryption up to 128-bits, making this kind of password recovery impractical due to the amount of time it would take. Many products can instantly remove 'owner' passwords as long as the PDF document is unprotected by a 'user' password.

There do not appear to be any freeware or open source .PDF password recovery programs easily available as of now. Several companies offer professional password defeating and recovery software products targeted at PDF files.

Here are a few examples which have trial versions available:

Elcomsoft's Advanced PDF password Recovery Pro

Lostpassword.com's Acrobat key

AdultPDF.com's PDF password remover

The GuaPDF program, like GuaWord, strips password protection from PDF documents by using a key search. Useful if the password is too complex to be revealed with a brute-force attack program. It also strips 'owner' password automatically.

A word about securing documents

If you are concerned about the safety of data that you have password protected, the best step you can take is to enforce complex passwords. While a password like 'sesame' can be broken a few minutes or less by most modern password recovery programs, something more complex like 'SeSam3E$eed' will take a prohibitive amount of time for any program to crack.

Note the inherent weakness of the security features of older versions of Microsoft Office and Adobe Acrobat to 'key-search' attacks. If you are using these programs and want to make sure your data is safe, strong passwords will not suffice.

It is also possible to use some of the third party encryption programs we list a little later in this article. Microsoft Office XP contains enhanced security features which make cracking the password by key-searching (matching the numerical key used to create the encryption) practically impossible. Please keep in mind that there is a balance here. The harder you make your passwords, the harder it will be to recover from a forgotten password. Professional services are available, but there is a limit to what they can do with a well-designed password backed up by solid encryption.

If you want to make sure that your documents are protected from other methods of password cracking, like key-searches, there are a couple of ways to do this. If you are using Microsoft's Office XP, Word and Excel documents can take advantage of XP's increased security features. These programs use 128-bit encryption, making the documents effectively impossible to crack by means of a key search except with a few super-computers.