As the title says I found a particular pdf in wich okular's review function is not working. Whenever I press F the review menu doesn't show, and by trying to access it via tools -> menu it looks like the function is disabled. Why does this happen in a particular pdf (all other pdfs I've read in okular didn't have this problem).
new123456's answer works, I used this method for a long time. The problem I found however was that in the translation from pdf to ps to pdf, the pdf's metadata and bookmarks were lost. A solution that doesn't lose this data:
$ qpdf -decrypt readonly.pdf editable.pdf
If you can read readonly.pdf without a password, but simply can't edit (or okular review) it, this will give you an editable copy of that file.
I had this issue with a pdf, Motorola 68000 Programmer's Reference Manual.
You shouldn't need to modify the pdf. To overcome this issue you only need to untick a checkbox in the settings. Go:
Settings -> Configure Okular...
Now untick Obey DRM limitations under Program Features in the General tab.
Click OK and restart Okular. You can now review the pdf like any other.