You can use wmctrl to add the flag demands_attention. This will highlight the window icon in the task bar. (It seems that you are looking for a solution like this, as Okular itself does not have tabs. And it's the thunderbird taskbar icon that is highlighted in your example.)
wmctrl -a okular -b add,demands_attention
As soon you bring focus in the window, it will lose the flag again.
You could write a script that allows you to select the window in the first place and then periodically raise the flag or raise it when triggered.
Here is an example of a bash script that lets you select a window, which will then be marked as demands_attention:
#!/bin/bash
HANDLE="$(xwininfo | grep 'Window id:' | cut -d ' ' -f4)"
wmctrl -i -a $HANDLE -b add,demands_attention
NOTE: This will only work if you disable the Open files in tabs option in the Okular settings.