That's very interesting to me. I remember a few times for tests in school where I could have told you the approximate page in the book the information was, the layout of the page, and precisely where the information was located on that page... but I couldn't tell you the information itself. Funny how the brain works!I prefer real, printed books for RPG use.
My personal experience matches some recent research* - the human mind typically learns better from a physical book, and can reference topics in a physical book better than electronic media. The physical book engages your ability to create mental maps to a far greater degree than a tablet or computer screen does. Combining not only the information, but where the information is found, tends to enhance the memory and ability to look up details. This effect is most strongly seen in long-form presentations, like in textbooks, or gaming books, as opposed to shorter articles.
So, for me, if I want to learn a new game, or if I'm going to be playing and may need to reference things in the books, the physical tome is superior. I will still use PDFs, but mostly for the cases where the physical book is not in print, or is otherwise unavailable to me.
*see, for example, the November 2013 issue of Scientific American.
Like many here, I prefer a physical book for reading through and learning. Touching the book and learning the layout, the art, the tables, it helps me remember much better. I can find information in the PHB, DMG, etc, without looking at the numbers because while I may or may not have the page number memorized, I know by the amount of pages/surrounding artwork, where the info is. I could fiddle around for five minutes in a PDF, trying to guess the page number and flipping through the thumbnails, but I could find the information in approximately ten seconds with the book physically in my hands.
I find reading PDFs for long periods to be harder on my eyes, and I remember less after reading them. Print it out, and it sticks in my mind better.
Again, like many here I do very much appreciate PDFs for referencing things on the fly, particularly if I'm DMing so I don't have to crowd my space with ten or twenty books. I would have loved to have PDFs of all my WoTC 3.0/3.5 books, because I do have the "stack of books" problem every time I DM.
I also appreciate PDFs for making player handouts, particularly for convention play so you don't end up have to share your book with a group of 4-6 people.