There are two products that you need to be aware of.
First, Adobe LiveCycle Policy Server is an ERM (enterprise rights management) product that allows you to control access to PDF documents, and permissions as to what a user can do with the document. So, in your scenario, you could use Policy Server to define a policy that could be applied to your PDFs. You can specifiy in the policy, who can open and view the document. The user must authenticate to the system with a user name and password to open the document. You can also set permissions at the user level, for example you could disable printing, copying etc. for UserA, and allow printing for UserB.
Policies can be applied to a PDF in two ways, 1) a user can manually apply a policy to a PDF using Acrobat 7.0x Professional or Standard, 2) Programmatically (automated) using the Adobe LiveCycle Document Security product
The second product that you need to be aware of is Adobe LiveCycle Document Security. This product allows you (through APIs) to:
Digitally sign PDFs (server side)
Validate Digital Signatures
Apply "password" based encryption (same password protection as Acrobat)
Apply Policy Server Policies to PDFs (server side)
Apply "certificate" (PKI) based encryption (same certificate based encryption as Acrobat)
For more information check out
http://www.adobe.com/securityRegards
Steve Forrest