I have enabled the features which are "open new files in tabs" and "switch to existing tab if file is already open", it works only if I open new files through the Okular application. What am I going to say is if I open new files just by double-clicking and the files will only open in new windows separately, it won't open in new tabs. Hope Okular Team can fix it as soon as possible and have a nice day. Last but not least, I did try the versions which are version 20.12.2 from Microsoft Store and 21.03.70 from nightly builds, and the problems still the same.