Look, I'm not planning on printing the whole thing, binding it and sticking it on my shelf. I want the pdf because then I can use it when I'm offline, across devices and search it easily.

If I want this book in hard copy, then I will purchase it - I've done this regularly with free digital books - but when it is offered free digitally then in my opinion prohibiting to only certain file formats is futile (as evidenced here), and such constraints are ineffective attempts to encourage people to buy the hard copy through inconvenience.

And I must add that this is no slight to the authors, whom have my greatest appreciation for compiling their vast knowledge into a book and offering it for free. These guys are legends.