I recently started buying paper books. Yes, it's not particularly ecological, but I found that I'm able to focus at the text much better this way. My Kindle (despite plenty of obvious advantages) just doesn't really work for me. It took me years to realize this, but I always start to tinker with the brightness settings, switch pages back and forth, go into the book library and back, play with highlighting words, etc.…
Paper books are also just orders of magnitude easier to flip through and re-read specific chapters, paragraphs or sections. I can skim through a shelf full of books in a way that I’m just not able to with ebooks, even with stuff like full text search available in Calibre.
Whatever works for you, of course. PDFs have bookmarks, which instantly take me to a specific spot on a specific page. Also, I can open multiple instances of the PDF, viewing multiple pages of the same book (or some PDF readers allow split screens to do the same thing). Edit: And if I don't recall where something is, I can search for it.