You can trash it if you like -- the update is fully installed already. If you wish to keep that file around in case you ever delete and reinstall Acrobat and want to update it manually, you can also do so.
It's completely up to you.
Adobe is sloppy in this way in that the auto-update mechanism leaves files on the desktop when updating. They should inform the user of what's happened, remove the disk image automatically, or prompt the user what to do with it (leave it or trash it)... unfortunately Adobe sometimes just isn't very Mac-like.