I’m a big fan of utilizing Rights Management Server (RMS) integrated with SharePoint when you have documents that need policies enforced not only in SharePoint, but at the desktop as well once the documents are downloaded from SharePoint.
However, PDFs have always been a bit of a challenge, as natively these are not supported. There have been some commercial versions of software out there for some time that extend RMS functionality to PDFs in SharePoint. Recently, a code source has been released on Code Project for a free version that is designed to work with SharePoint Server 2007 and SharePoint Server 2010.
Use at your own risk, etc… but it’s definitely worth taking a look at. http://pdfirmprotector.codeplex.com/ by Adam Buenz.