Is the PDF/A mode active?
Thanks for the tip, I just disabled PDF/A mode through Edit-> Preferences-> Document and now all the editing tools are enabled.
I have the same problem (Highlight and other tools grayed out, no security restrictions in place), but when I try to find Edit>Preferences>Document to turn off PDF/A, that choice does not appear in the menu. The only mention of PDF/A in the help guide for my version of Acrobat is in "new features" and it does not tell me how to find this. How can I enable editing? I did not originate the document.