My assumption is that the comment I was responding to addressed Google Docs specifically (which afaik is web only?). If this is the case, then your options to run locally would be for the conversion to run in your browser, or for you to have some Google agent run on your computer that can handle these requests instead of the Google servers themselves, neither of which is a scalable option in this case (due to browser differences / expecting people to be able to install software locally to their devices)?
Surely a file conversion should not be affected too much by browser differences, it should be a pure function, pure calculation not requiring too many APIs and which doesn't have much to do with rendering.
