I sometimes use Okular to annotate a PDF using highlighting, drawing boxes, underlining or writing notes. This feature is called review in Okular. However, I noticed that this review feature is disabled on certain PDF files. I did check the filesystem and the file was not readonly.
Turns out that when a PDF has DRM options set in it, Okular obeys those limitations by default. Thankfully, this can be turned off easily. Go to Settings → Configure Okular → General and uncheck the Obey DRM limitations option. You will need to restart Okular after you apply this. You will now be able to review the PDF.
Tried with: Okular 0.24.2 and Ubuntu 16.04