Prevent users from opening sensitive documents by password protecting PDF files.
Most PDF creators can be used to add security to documents. A good free program I use is PrimoPDF - it installs as a 'virtual' printer on your computer and allows you to create pdf files by 'printing' documents to PDF. It will convert to PDF any file you can print. In the process you have the option of adding a password to protect access to the file. You can also prevent people from copying text and printing the PDF.
Try opening this document - you could also try downloading the PDF but you would be unable to open it without the password...