Adobe Acrobat Encryption and Security History
The Adobe™ PDF document format has been in development since 1991, and from as early as 1994 included security features that were aimed at preventing users from being able to make changes to a published documents. This involved the use of encryption as the only practical way of protecting information. The basis of PDF encryption is to prevent users viewing the file if they are not authorized, and if they are authorized, to control what they can do with the file (i.e. whether printing is allowed, etc.).
Early PDF document security relied on weak 40 bit encryption and soon after it was released methods of breaking it were freely available on the Internet. In 2001 128 bit encryption became available to prevent simple hacking of the native mode controls, and degraded printing, was also added. In 2008 256 bit encryption was added to Adobe 9 but the implmentation was not as secure as Adobe 8’s 128 bit encryption (the password checking routine consists of just one call to the SHA256 hash function) enabling passwords to be cracked a lot quicker.
Password recovery software is able to break Adobe PDF encryption since common PDF security flaws still undermine Acrobat PDF security to this day.