2 pieces here.
1. Dropbox - Free - Syncs up documents and other files, up to 2 GB for free. I sync between 4 machines including my WinXP laptop at work, Win7 Laptop at home, MacBookPro at home and my iPad.
2. Documents to go Premium - $15 (watch for sales) - The Premium version interfaces directly with Dropbox. It is directly compatible with Microsoft Office files (Word, Excel, Powerpoint) and will allow you to read/view/edit many other document formats.
For #2, you will probably get a lot more suggestions, all of which are good in their own rights. D2G has been around since the Palm Pilot back in the late 90's. They have a great track record of a quality product. I have used it on different devices and it just works. I have tried the other Office products on the iPad and keep going back to D2G.
Use the app Appshopper to watch the prices for you and alert you to price drops. I ended up getting D2G Premium for $8 instead of the $15 when they were doing a sale.