There are several free PDF viewers out there, but I don't know what sort of limitation there might be in finding one to run on the iFly's platform. You probably already know this, but Wikipedia has a convenient list of free / open source readers (here), and this article has a brief pro/con review of several free readers.
Are there any particular technical deficiencies you've noted in the free readers you've checked, or is it more of a licensing issue (free for individual use but not commercial use, etc.)?