It may be down to the viewers you have for PDF files, but here at least I can get the preview panel to show what I like sizewise – the whole page, fit to Windows or whatever. I can even copy text from the preview and use that to rename files in Opus.
This behaviour at my end may be down to the fact that I have Quick View Plus installed. If you cannot do what you with Opus as is, or with some other free addon, perhaps buying Quick View Plus will achieve your goal.
I may be wrong, but I doubt if there is going to be any way of renaming a file when it is open in its "native" application.