Documents To Go
If you handle a lot of Office files, Documents To Go may be just the thing for you. The free version gives you the ability to view Word and Excel files. The full paid version adds editing capabilities, along with PDF and PowerPoint viewing options.
Note: This link takes you to the vendor's site, where you must register to download the version of the software appropriate to your supported device. Prices and versions vary by device; the price given here is the price for the full version for Android.