How to open a *.pdf in an already existing instance of Okular (independent of in which workspace that instance resides)?
Solution 1:
I recently wanted to have the same functionality. Here is the simplest solution I could find:
Open Okular GUI. Navigate to Settings > Configure Okular. In General, under Program Features, check the 'Open new files in tabs' option. Click Apply. Click OK.
Then on your terminal, run okular file1.pdf file2.pdf fileN.pdf.
That code should open all your files in the same Okular window in different tabs. This method works across folders.