Why won't Adobe add Save As option like they have on the desktop version. Very annoying they haven't done this already. I know I'm not the first person to ask this question.
File > Save As... is available in Acrobat Reader DC.
I am not sure if this is only available for Acrobat Reader Mobile for AndroidOS devices.
"Save a copy" was included in the last update and is currently featured.
See if you need to bring your Acrobat Reader mobile app to its current version update.