I think it's funny that the top two comments of this post are diametrically opposed, yet I kind of agree with both of them.

The PDF is a terrible format, yet if I'm sending an email with an attachment I want you to see exactly how it looks on my computer then I'm exporting to PDF.

However if your book is only available as a pdf I'm probably going to skip it.

PDF is good for short things, a contract maybe. The best use case is forms which this doesn't really talk about but seems to address, the web has basically solved it, but there are times you want to send people a form to fill out that you don't want the formatting to be go wacky on, but still need to be editable.

PDF can do this but isn't good at it, this seems to take that not good and make it good.