I recently upgraded to Ubuntu 16.04 LTS from 14.04 LTS, and the Okular version now is 0.24.2. Now I cannot annotate certain pdf files (the "review" option is simply greyed out), which I previously had no problem annotating and viewing the annotations of.

The PDF file is just some lecture slides in a course. According to "Properties" tab in Okular, this PDF file appears to be produced by Acrobat Distiller, and its "security" shows "encrypted". But apparently those didn't stop me from annotating it before.

Am I missing something to install alongside Okular? How can I annotate such a pdf file now.

Maybe https://bugs.kde.org/show_bug.cgi?id=341999 ?

Thanks. It seems that option was disabled in my previous installation or I turned it off but forgot I did it. Now I can annotate the file again.