A professor accidentally sent me a copy of my letter of recommendation for grad school. What should I do?
You still need that hardware letter so I guess some uncomfortable situation may have to happen. Yet, I think some awkwardness can be avoided by focusing on the application. I'd suggest e-mailing him and tell him that it appears a wrong attachment was sent to you, and invite him to double check if the hardware approval letter was uploaded to the school application portal, follow by a polite ask for the actual approval document. End with a statement saying that you'll delete his previous e-mail and the attachment, and thank him for his help.

If you have not read it, then delete it. That'd be the best scenario. If you have read part of it by accident, notice that you have done nothing wrong. You simply waived the right to read it; you did not signed a contract to never read it. Yet, don't try to read all of it even it's not your fault, it isn't meant to be for you to read, and knowing the contents may affect you, even you think you can be completely objective.