path access limit access to files directory

I'm trying to use path access module to restrict access to paths like sites/default/files/xxxxxx/yyyy.pdf.
I can't seem to put it to work...can anyone help?
I've read (in admin>user management>path access) that "Each user role can be granted or denied access to any url paths" and "Page access is not limited to node pages only, anything can be controlled using paths. "..What am I missing?