Documents to Go for iPad
Docs to Go doesn't improve the rendering of the .doc or .ppt in Safari on the iPad, but it  does register with the system as a compatible app for the file type. Tapping in the document area reveals an action bar with two buttons: Open in  (which depends on the apps you have installed), and an Open in... button, which when tapped, gives you a popover with compatible apps. 

See this example:



In my experience, you would be better off using Docs to Go to render the documents in any case, as the fidelity of its Office document rendering is quite good.