Originally Posted by kiwidude
I'm sure someone will correct me if wrong but I don't think there is no way to do that without a custom plugin, like the "Open ePub in Editor" one. I think you can only configure Calibre to either use it's internal viewer or else it will use your system default. It would be a trivial exercise to write a PDF menu item based on that ePub plugin code though.
A custom plugin based on the "Open ePub in Editor" is what I suggested to him in another thread. That is exactly why he is asking in this thread.