My PDF viewer says that something wrong, but File Explorer allows to preview with no issues, so I guess it's not issue of PDF handler.
I have both Adobe and Adobe Reader installed and they work fine. Any guess how that can be fixed?
Try reinstalling Adobe Reader.
If you have installed PowerToys and turned on its PDF viewer, turn it off again (it's terrible and blocked in Opus as a result, which may be why it's showing PDFs in Explorer but not Opus; we'll improve how the blocking works in the next version).
Dear Leo -
I've turned it off in PowerToys and it started working. What a relief. I suppose they've activated it on version update as I do not remember myself doing it.
Thank you for the assistance. I've not reinstalled Adobe Reader, there was no need for this.