You could probably keep it open-source but charge for the print copy and pay-what-you-want for the PDF.
currently the book is on github and gitbooks, so I doubt if anyone would pay for the pdf :-)
Started making a book on leanpub.
The premium version of Gumroad is $10 per month, and reduces the cut they take.
There's leanpub too, though I don't know if you can upload an already made pdf. Worth checking out though.