restricting pdf from copying/sharing

Acrobat has several different types of security methods -- providing a wide range of capabilities. They are: password security, certificate security, and LiveCycle Policy Server security. Password security is a simple way to share documents where password sharing is possible. The password security encryption algorithm varies by security type and product version (i.e., 128-bit RC4, 128-bit AES for Acrobat 7 or 8, 40- and 128-bit RC4 for Acrobat 5 and 6). RC4 encryption is used in a wide variety of applications including WEP (securing wireless networks) and SSL (secure socket layer). The AES (Advanced Encryption Standard) is also widely used but is a based on a different encryption model. Both contain inherent strengths and weaknesses.

However, if your goal is to restrict copying and sharing of PDFs, then you'll need to look beyond password security to either certificate security (if you have a controlled user environment) or LiveCycle security. Password security doesn't control unauthorized copying or sharing of PDF content.
For a more detailed discussion of the security features in Acrobat you can check out the [url=http://www.adobe.com/devnet/acrobat/pdfs/doc_security_user_guide.pdf]Document Security User Guide for Adobe Acrobat and Adobe Reader[/url]