I have Fedora 31 KDE spin (Gnome desktop is not installed). Okular is the default application set for PDF files. STEPS TO REPRODUCE 1. Open KRunner by Alt+F2 and type "pdf" in the box 2. Click on any PDF file from the search result in "Document" category 3. The file is opened in Gnome evince The problem is only with KRunner results belonging to "Document" section. PDF files in "Recent Documents" section open in Okular. When opened from Dolphin or by xdg-open, PDF files are opened in Okular, as expected. In File Associations in System Settings, Okular is the first handler for application/pdf and evince is second. This is possibly related to a particular KRunner plugin. I couldn't correctly guess it (file search or places, probably).
Today, I tested it again and it is working like expected. PDF files are getting opened in Okular, as expected. I have no idea how this got fixed. I had installed several updates from Fedora over these days. So no idea what fixed this. Closing the report for now.