There are two, really.

First, really simple interface - you only need to send e-mail. If you don't have a PDF ready, we can even generate one for you from the email content. This may sound trivial, but I think learning an additional app is one of the two key factors pushing people away from these solutions.

The second USP is the free tier, with no sign-up, etc. Again, this may sound trivial, but I think this turns a large fraction of potential customers away.