Recently, when I view a .pdf in the Viewer Pane, I'm not seeing a full-page view and there isn't a scroll bar or page count at the bottom of the viewer pane. Is this something that has changed in DOpus or in Acrobat (and, if the latter, does anybody know how to change the behavior so that the Viewer Pane will show the full page)?
PDF viewing is down to whichever viewer you have installed. It's not part of Opus itself.
If the wrong PDF viewer is being used, reinstalling the one you want to use is usually enough to fix things.
(You should also see the same thing in File Explorer's viewer pane in most cases, since it works the same as Opus: The PDF viewer isn't part of the file manager itself.)