(disclaimer: one of the Dangerzone devs)

That's something I do from time to time as well. AFAIK Google Drive renders all documents on the server-side (which implicitly means that they don't trust the browser sandbox), so that's a reasonable price to pay for less privacy.

Dealing with sensitive documents though is another story, you just can't upload them to a third-party service. That's where projects like Dangerzone come into play.

Does google drive apply any transformation over the PDF, or are you effectively loading the same document in your browser on the round trip?

Do you have any specifics on what Drive does? Any examples of it fixing embedded virii? Or is this blind assumption?

Firefox has a builtin PDF reader, PDF.js, that resides inside of the Javascript sandbox. In theory, it's as safe as loading a webpage.