Using the tablet version of Yahoo Mail's web client: for .doc files (newer formats may not work) you can tap on the document and it will open in Safari. If you tap in the center of the screen you will get some menu items at the top. On the right you should see an Open In (some app) and Open In. You can tap on these to open the document in a compatible app like Documents to Go.
This will probably work with many common formats, but Safari does not recognize all extensions, and the ones it does not recognize will probably not work this way.