I wrote about an Adobe zero-day exploit a while back... They've been in the security news quite a bit lately; their huge success with Acrobat as a document distribution standard and the Flash media player becoming more and more common has ironically made their products a favored target of malware creators.
I finally got fed up with how large Acrobat Adobe Reader has gotten - the version 9 installer for XP is 35.7 MB (vs. 21MB for v8, 15MB for v6, ...) - so recently I installed a free alternative called Foxit. The latest version, 3.1, has a 5MB installer that results in a 7.2MB installation. Compare this to the 206MB Acrobat Reader installation that is on my PC!!! (wtf?)
But the best news is that Foxit starts up much faster than Acrobat Reader. I view PDFs all the time, and so far I have not had any issues with this application. It does have a tiny advertisement window as shown below (The blue area in the upper right corner), but this only seems to advertise Foxit's own payware products, which seems fair given that the reader is free.
I'll also mention that Foxit is potentially a better choice than Acrobat Reader since most exploits that are targeted at Acrobat do not manifest themselves in FoxIt. However, Foxit's popularity as an Acrobat alternative has made it the target of the malware authors, but it appears that the Foxit folks take security seriously and are prompt to release updates, as discussed here.
For the moment Foxit is available for Windows, Linux, and some handheld OSs, but not the Mac.