If you just want to display text files, look at the fpassthru function, but if it's something exotic like microsift word you will have to do some fancy stuff e.g.
convert the .doc file into HTML with a 3rd party tool
allow word to fire up on the client PC to view it
Look at the ms office section on the microsoft web site which have ofice file viewers to download (which allow you to view word and excell file without having the full product) which you could include a link to for users to access if the need to.