XML is a bunch of ingredients, PDF is the finished meal.
He or she who knows how to cook can create a wide variety of meals using the same ingredients. With a potato, he can create soup, mashed potatoes, crisps, french fries,... There's an almost endless list of possibilities.
He or she who can't cook, will stare at the potato and wonder: How on earth can I turn this ugly vegetable into a nice croquette?
The answer is: you need a recipe. That recipe could be an XSL:FO file, the XHTML specification, a DocBook implementation, an XFA template,... Without that recipe, you'll never be able to turn your XML into PDF.