LC Rights Mgt (now called AEM Document security) is an enterprise product.
It's a policy server which integrates with your organizational directory, and then members can create their specific policies restricting access to specific users on that LDAP.
The same policies can then be applied to PDF documents, by connecting Acrobat with your policy server.
No plugins or extensions are needed if you are opening LC protected PDFs in Adobe Reader/Acrobat. I have not had success opening protected PDFs in any other viewers other than Adobe's own products. An extension is required for MS Office files (32 bit office only). Also - there are many options for making the documents expire when you want them to, globally or by user.
correctly stated by nick. usually Adobe rights management works for PDFs, but you can get plugins to get protection option in MS Office as well.
You only need the plugin if you want to protect or read MS Office files: