Your adobe acrobat reader version 15.3.1 for android don't have option to remove recently opened file one by one. So please update this feature in next update.
BECAUSE YOU ARE NOT UPDATE THE WINDOW APP WHICH PROBLEM I POSTED U IN PREVIOUS ASK. THAT'S WHY I PURCHASE NEW ANDROID MOBILE
Unfortunately, we currently don't support this feature. We have taken note of this as a feature request, and would try to incorporate it in the application in our future releases.
Adobe Acrobat Team