You can look up an invoice issued to a client in 2 ways.

If you don't recall all the details of the invoice, you can go to the Monetary Report, select the time range and at the Sales part there will be all the invoices sent  to your clients.



Or the second option will be to search by the order number in the Orders Section from the Menu, and then extend the order and you can see the invoice, in case you will need to reprint it.