Hi ,
You could directly delete the document you downloaded by right clicking on the document and selecting delete.
If you want to delete the it from reader ,then there is no such option available.
All you can do in this case is clear out the documents in the recently used list by setting the number to 0.
Launch Reader>Navigate to Edit>Preferences>Documents>Set documents in recently used list to 0.
Regards
Sukrit Dhingra