True, but there's no delineation provided in the PDF files so as to ascertain which files require which features (via file extension or whatever), and so it mig

True, but there's no delineation provided in the PDF files so as to ascertain which files require which features (via file extension or whatever), and so it might be impossible to tell if your particular reader supports all of the features provided by the PDF file. The better readers will tell you that some features of the current document aren't supported, while others simply won't render correctly at all, or worse, will just crash the application. You also end up with users trying to open some dynamic form PDF with scripting and whatnot enabled, and of course your portable device balks because it doesn't support it, so now it gets kicked back to the device MFG because the PDF reader is "broken".