I had a trailing forward slash at the end of the absolute path. That is what was causing the double forward slash in the URL. I restarted Apache, and now I'm getting a blank page instead of the no preview available error. Update: So much for that. Now I'm back to no preview available.
The Google URL now is
https://docs.google.com/viewerng/viewer?url=https://www.stovebolt.com/ubbthreads/uploads/attachments/43462.pdf
I have confirmed that the file exists and has RW perms.