As it happens, Adobe’s Acrobat PDF(Portable Document Format) program has overtaken Microsoft Word as the program most attacked by viruses.
This is because Microsoft has been trying very hard to fix any and all of the loopholes in Word and Office that allow viruses to attack them, while Adobe has not.
This has become a problem because more computers have some version of Adobe Acrobat on them than have Word, and most installed versions of Acrobat are old and unpatched; this allows loopholes in the program to be exploited by the bad guys.
See the article here: http://www.theregister.co.uk/2010/03/09/adobe_reader_attacks/
If you would like an alternative to Acrobat, try the free Foxit PDF Reader: http://download.cnet.com/Foxit-Reader/3000-10743_4-10313206.html