Once a document has been sent (there is a date visible in the 'sent date' column) it cannot be deleted.
However, if you are the sender of the document you may choose to 'hide' a sent document so that it is no longer visible within your programme by following the steps below:
1. In the Documents section of your OP (1) select the document from the list (2) and click the Edit button (3):
2. In the Document Details pop-up select the document you wish to hide (1) and click on Update (2) to confirm:
3. The hidden attachment will no longer be visible in the document list: