My friend Patrick is very much against PDFs. He is right to be. I think the PDF of a book is best thought of as being a bit like a "Greatest Hits" album or one of those awful "Chilled Classics" compilations: a sort of thin, scraped-off surface film from the top of a stew that has none of the nutritious content of the meat and veg underneath. The content of a PDF may be the same as a book, but people only interact with it by skimming off that top layer. Scroll up, scroll down. Look at the pretty pictures. Read a paragraph here or there. Press Alt+Tab. Check what's going on on Facebook. Alt+Tab back to the PDF. Scroll up, scroll down. Look at the pretty pictures. Read a paragraph here or there. Press Alt+Tab. Check you email. Etc. This is not people's fault. It's the technology. The PDF is useful for easy distribution. It is bad, very bad, for actual sustained reading.
(At this point somebody will chime in in the comments about how they prefer to read books on their PDF and have no problem focusing while reading a book on screen. Good for you - really, I don't mean that at all facetiously. But you are unusual.)
There is another reason to be against PDFs. Imagine you had to predict whether it is more likely that books or PDFs will be defunct technologies in 100 years' time.
The answer is PDFs. Books have been around for an exceedingly long time. They've proved that they are robust to technological development and social change of great magnitude. PDFs have not. I make no predictions, but given that nobody has any idea what will happen over the course of the next 100 years, you have to back the humble book to last the course. And if you want to create or even merely possess things that will last, go paper.
The final reason to be against PDFs: things made from trees are beautiful. I sit here on a wooden floor with my laptop resting on an opium table made from mango wood. Next to me is a book made from paper that feels and smells gorgeous. Don't buy into the anti "dead tree" hype. Beauty matters.