SUMMARY Okular (and only Okular) cannot open PDF files from kdeconnect mounted devices. Zathura and Firefox are able to do so, and I can copy said file to proper local storage and then Okular can open it. STEPS TO REPRODUCE 1. Have a pdf file on your kdeconnect-ed phone 2. Browse to it via Dolphin 3. Try to open the pdf file in Okular OBSERVED RESULT Okular fails to open the file with message "Could not open file:///run/user/1000/device-id/sdcard/test.pdf." EXPECTED RESULT Okular should be able to open the file. SOFTWARE/OS VERSIONS KDE Plasma Version: 5.20.4 KDE Frameworks Version: 5.77.0 Qt Version: 5.15.2 ADDITIONAL INFORMATION Other KDE apps can open their respective files via kdeconnect with no problem (Kate can edit text files, Gwenview can view images, Dolphin can browse).
Interesting, that URL points to a FUSE mount.