Hi,
With the latest release, the 'Clear Recents' button has been removed. However, You can delete the recently opened documents by following these steps:
1. In Adobe Reader, go to 'Documents' tab.
2. Long press on the file(s) you want to delete. The top toolbar options will show the number of files selected.
3. In the top toolbar, the delete icon will also appear, which looks like a trashcan.
4. Tap on this option to delete the selected files.
Please note, that after following these steps, the file will be permanently deleted from your device and cannot be recovered again. Please let us know if this is not a viable option for you.
Thanks.