Would a client be able to view deleted invoice and attachment emailed to them earlier?

The client would still see the invoice and attachments on their email, even if the transaction was already deleted in QuickBooks Online, David.

Please note that deleting a transaction does not recall or reverse the sent email. However, they'll get a message that says "Invoice deleted" once they click Print or Save.

Feel free to check out this article in case you want to restore an accidentally deleted transaction: Use the audit log to re-enter deleted transactions in QuickBooks Online.

Please know that the entire forum is always ready to help if you have more questions for us.