A very good document hosting site that I have reviewed before is Scribd. It is going to get even better. It is transitioning from Flash to HTML5.
HTML5 will run on iphones and ipads, unlike Flash. Plus it will be more expansive taking up the whole browser rather than sitting in a little window. Plus, it will download faster. In addition, the service is about to introduce compatibility with Google Docs. this will make Scribd much more usable and useful.
For more, see:
Scribd: HTML5 & The Future of Publishing