Since the version 0.19, Okular has a tabbed interface. But in my version (0.22.0) I don't see the "Open in new tab" option in the settings (as explained in the Okular documentation). I precise that my installation is under a Gnome environment. What should I do to make the tab feature working ?
You have to enable checkbox "Open new files in tabs" first in the Okular settings. After that "Open..." command will open new file in tab instead of new Okular instance.