Okular doesn't allow me to do reviews on certain pdfs

I believe that there are PDF's that have flags that set them as readonly. Use

pdf2ps foo.pdf; ps2pdf foo.ps foo.pdf

This uses Ghostscript to print the PDF into Postscript and then back into PDF, thus circumventing the readonly mechanism.

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.