Soda PDF is a free PDF reader (in a crowded field) that renders a PDF as if it was a 3D magazine. One turns pages by 'pulling' the page across in a realistic simulation of an actual page turn. While emulating a print publication in such a literal way might seem retrograde, there is something satisying about the reading experience, and it is an aesthetically pleasing way to preview design work (particularly files destined for print). The effect renders very quickly and smoothly, and can be turned off if you want to go back to old fashioned flat viewing.