Found tips that say to go to prererences/viewer/plugins/(more stuff). My line ends at plugins. No further options. This feature has worked fine for me for the last decade or so. Frankly, it should be easy.
Is your PDF viewer working in File Explorer's viewer pane?
NO. not working in OPUS or File Explorer. I do have Adobe Reader, Adobe Acrobat DC and Kofax Power PDF. Can't find a way to link them into the file programs.
Try uninstalling and reinstalling Adobe Reader.
Uninstalled Reader. Reinstallation gave me an error message that Reader was still there. Ran a cleanup tool from Adobe. Then reinstallation did work. Still, no change in D. Opus viewer panel.
If it's not working in File Explorer's viewer panel either then the issue is with Adobe Reader (or something else on the system interfering with Adobe Reader), and outside of Opus.
All we can suggest is reinstalling Reader (which you've already done) or trying a different PDF viewer, or asking Adobe for support if you need to use Reader specifically.