All of the computer magazines advise that Adobe Reader is one of the (many) programs that nobody should ever have on their computer. Foxit (as recommended by Lie-in King) is by far the most popular alternative but Sumatra PDF (which is what I use) is also excellent:
http://www.sumatrapdfreader.org/free-pdf-reader.html
Adobe Reader is cumbersome and slow (especially on older computers running XP). If you click to download a pdf file, not realising that it's several hundred pages long, you can end up waiting ages (perhaps a quarter of an hour or longer) for Adobe Reader to fully open it. Sumatra PDF will open it within a few seconds.