Good question. I can open the PDF in the Andriod App, and I can even see all of the highlights I made on my PC. The issue is when I try to highlight more text in the App. Then I get the error message. That is really frustrating that this is still an issue!
Sounds similar another question posted (I just found out today link below ) There some insite which might help.
https://community.adobe.com/t5/acrobat-reader-mobile/highlighting-text-using-adobe-reader-on-ipad/td-p/8840859?page=1
Good to know that much like the fill and sign problem. If Your PDF document is encrypted. Acrobat Reader for iPhone and iPad does not support saving changes to encrypted PDF documents. This is a limitation in Acrobat Reader mobile apps for iOS, Android, and Windows Phone.