Are you a developer? Do you want to add "PDF Encryption", "PDF Decryption", "Set the permission of the user such as printing, modifying etc" feature to your application?
Now, you can try PDF Security ActiveX DLL, with a few function calls, it helps you decrypt & encrypt pdf in your Win programs or Web applications.
PDF Security ActiveX DLL was intended to Protect Adobe Acrobat PDF files, that comes with "owner" and "user" password set, preventing the file from editing (changing), printing, selecting text and graphics (and copying them into the Clipboard), or adding/changing annotations and form fields.
All editions of Adobe Acrobat (including 5.x, 6.x, 7.x, 8.x, 9.x which tools 40-bit,128-bit RC4 & AES, 256-bit AES encryption) are allowed by PDF Security ActiveX DLL.
It is a standalone component and does not depend on Adobe Acrobat, or even Acrobat Reader.