The first thing is that you must have a paid subscription of Adobe PDF Export pack or Adobe Acrobat Pro DC.
You need to sign in in the Acrobat Reader mobile app with your Adobe ID to unlock the Acrobat premium editing features with the mobile app, or to use the Acrobat online services for file export.
The steps in an Android device are very straight forward.
If by text file you mean like a MS Word document, you need to tap and long press on that file icon for a second or so, and a context menu will open up with the option to convert the file to a PDF.
Just remember, you must be subscribed to Acrobat with a paid-for plan, and also be signed in with the Acrobat Reader mobile app in order to be able to export a file to PDF. Otherwise you won't see these features available until you're signed in.