Acrobat Bookmarks are not visible or greyed out

Many questions about bookmarks not showing or greyed out in Acrobat. I've seen may responses just showing normal way to see bookmarks tool bar etc. that don't work. 

What I found that was causing bookmarks to not be shown (hidden) and not being able to add new bookmarks was that the .pdf doc was signed. Even if there is no security set, once a .pdf is signed it's protected and bookmarks are not available and new bookmarks can't be added. 

To bypass signature save .pdf as a WORD doc. Then save new Word doc as a new .pdf and bookmarks are once again usable. Signature will still be visible as on original .pdf.