I think the point is that if you are designing with that size in mind you may make different decisions about the column layout etc. I would have thought that a

I think the point is that if you are designing with that size in mind you may make different decisions about the column layout etc. I would have thought that a PDF of a book would normally be made in the size of the physical book, which could be A4, but usually isn't (at least not when I look at my bookshelves).