Please find the details below on how to delete folders from Acrobat Reader mobile.
If you are trying to delete folders created in Document Cloud storage than Tap & Hold the folder which you would like to delete.
Once the folder is selected it will highlighted in blue.
You can tap on other folders also to delete multiple folders at once.
If you are trying to delete folders in Local or Document Cloud storage than tap Local or Document Cloud.
You can also refer to these links for managing file on Android & iOS.
Let me know if this helps.