Could you please confirm the version of Acrobat Reader installed on your phone.
For that, launch Reader>tap 3 vertical dots in the top right corner of the screen>Help.
Tap three vertical dots again and select About Adobe Acrobat
Btw, it is not on a phone, but on a three year old tablet, if that makes a difference...
The tablet is running Android version 4.1.2
Try to clear cache, restart the tablet and see if that changes anything.
Tap Settings on your tablet's home screen >Tap Apps>Adobe Acrobat>Clear cache.
It seems to have fixed itself... but I cleared the cache anyway... hope it stays fixed...
Thanks for your help.