I have read studies which conclude that we retain more when reading paper books instead of a PDF on a screen.
I have found that it is because we interact with the book as we read, and all of the interactions/associations create a strong memory relative to pressing a button or swiping your finger across a screen.
I have also read something claiming that we read from screens in an "F" pattern, and we read each line fully with a book.

What do you think?