I recently updated 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.
According to "Properties" tab in Okular, the 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. It's just some lecture slides by our lecturer.
Am I missing something to install alongside Okular? How can I annotate such a pdf file.