The kindle is roughly the size of a paperback... I can't imagine there would be much difficulty in making a pdf display in that size (and I routinely print pdfs at 2 pages per page side).
Many PDFs are made in letter size, so they would get scaled down to the point of unreadability on the Kindle screen. I don't think it's fair to compare the Kindle screen to 2-up paper, since printers have much higher resolution.
