If your on the go in an office environment, PDF files are a must.
I work for the Marine Corps, and use a lot of PDF files, but sadly our computer systems are contracted out to EDS. So no one has rights to install things, and on some machines Adobe isn't there. Sometimes that just puts us in between a rock and a hard place.
GhostView is a great opensource PDF viewer. I believe it would be well worth the time and effort to make it portable. Also why not include Ghost Script so people can also create PDF files. The cost of the Adobe program to create PDF files is outrageous and would be benificial to a lot of people.