I'm happy to provide you the helpful information about retrieving attachments sent in the invoice.
In QuickBooks Online (QBO), we're unable to retrieve the attachment if you've deleted the actual files. However, if you delete the invoice with the attachment, you can still view it in the Attachments list.
Here's how to view it:
Go to the Gear icon.
Select Attachments.
Then, see the lists of attachments from all of your transactions.