The difference between the XDP and the PDF is that the XDP has the
template definition (the XML that describes how to make the form) and
the PDF has a PDF rendering (as well the XML template). This is why
the PDF is much larger than the XDP. As for when you would use one
over the other, typically you would use the XDP if you were going to
be using LiveCycle Forms as part of your process for rendering the PDF
version of the form on the fly. If your form was dynamic or you
wanted to prepopulate it with data, you might want to do this. If you
are not using LiveCycle Forms, then you would want to save the PDF
(rendering) as the server component (LC Forms) will not be there to do
that. The samples are made with the assumption that a customer has
access to all of the different LiveCycle components which is why you
see examples using both. I hope this helps explain things a bit
more.
Dave