If a file is loaded for viewing, it is stored in the cache on the client computer from where it can be recovered. While it is on the screen, the content may be also captured as image.
You can protect the PDF document with a password and disallow copying, editing, and printing.
This question is not a new one. Using Google with 'prevent saving pdf' gives you millions of results. A good one is this: http://answers.acrobatusers.com/How-I-disable-ability-user-save-copy-PDF-The-file-online-network-drive-q2454.aspx