Jakob Nielsen says that you shouldn't open "non-web documents" within the browser window. Some of our users had the same complaint and were asking for us to prevent pdf, doc, and other file formats from opening inline within the browser window.
The way to get this to happen is to add a "Content-disposition" header with the value of "Attachment". Of course if you are using Rails send_file or send_data methods you can easily add this header. If, however, the files are being served up directly by your web server then you need to find another way to add this header. If you are using Apache 2 the following directives in your conf file should do the trick: