update Word/pdf file over internet ?

I am build a web site, on which I want to let the user edit & save a Word or
PDF document, which is on our web server.



I know I can open a file across the net (ie. winword
www.testurl.com\myfile.doc).



But what do I need to do on the server to let the user save his changes.



Also, are there security implications. Can I have password protection, to
stop people changing the file, unless they are on my web site.



Any suggestions please.

thank you.



Garry