Why not just send a message or post a backers-only update with a link to download the PDF? That's how we do it. No emails needed. That's how we always fulfill digitally within seconds of a Kickstarter ending. We have the backers-only update ready to go, and we message the backers with the link. I can't remember offhand whether Backerkit offers backers-only updates or backer messaging, but I'd be surprised if they don't. I don't recall us having any issues when we ran Backerkit projects.