If you have deleted a receipt or other document, you can recover it by tapping Menu>Trash. There, you can swipe from left to right to “restore” the document to the original folder where it was saved.
For the future, if you want to archive receipts but may need to refer to them later, instead of deleting the receipts, you can choose to create a folder and name it something like “Trash” or “Archive” and save receipts there instead of deleting them.