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GroupDocs provides a commercial online viewer and API's for supporting those formats as well as .pdf

That is awesome! link for the lazy: http://groupdocs.com/

http://groupdocs.com/ seems to cope with a whole bunch of file formats.

Is there a way you could tell your browser to open those links in google docs by default?

Proxying all your documents to Google is something that should be avoidable.

True. And possibly undesirable depending on the sensitivity of the files.

Maybe Web Intents?