Secure Document Proposal

Recently a group asked me to create a folder within their datasource to use for a high-security sub-project.  I worked with them to set up a top level folder that only their group can view and access, but there is still some concern about file security and secrecy.  The issue is that even though we can give users read-only access, the file is still kept locally on their PC.  Yes, I know I can have the local file purged but that doesn't prevent someone from copying the local file while it's opened.

I was thinking that for security it would be helpful to have the ability to lock-down files so they can only be viewed directly from the ProjectWise server - no file transfer takes place.  Thoughts?