A: Download PDFs, not the software they come with.
The PDF format is an open standard, which means that it is not owned by any one company, and anyone can create software that can read and write PDFs.
There are many PDF readers available for different operating systems. Some are free, and some are not.
On Windows, we recommend the free PDF reader from PDFForge.
On Mac OS X, we recommend the free PDF reader from Skim