PDFs can't be displayed
Some jpg's do not dispay
You can choose the colour for the Viewer pane background but you can't change the font
PDFs can be displayed using the same PDF viewers which work in File Explorer. One comes with Windows these days, so every up-to-date Windows 10 or 11 system should have one (if it hasn't been broken by other PDF software, which is fairly common as they fight over registry settings).
If some JPGs don't display, please zip some examples and send them to us so we can look at them. I suspect they are not really JPEG files or are corrupt in some way, but we can look into them. Please start a new thread for that. ( Ask one question per thread )
Text viewer font can be configured in the text viewer plugin's Preferences. The fonts other viewers use depend on the viewer, and those viewers may not even be part of Opus if we're talking about something like PDF.