Is it possible to print directly to the fax service? That's what we've did once for a very similar service: Provide a server using the Internet Printing Protocol (http://en.wikipedia.org/wiki/Internet_Printing_Protocol) that clients configure as remote printer (this works fine in Windows and Linux, we didn't test OS X at this time). The things that a user has printed are stored in an "outbox", where the user can just click on an item and specify where it should be sent. The advantage of this method is that you don't need the upload step and that you are able to support any application (without requiring users to convert the data to PDF first).