It cannot be done. PDF is a final layout. Unlike the web page, where you are never sure what you're getting, the whole purpose of a PDF is to look the same no matter what device, or even medium, you're accessing it from. It basically says, "there will be the phrase 'Hello, World' in this font, this point size, at these x and y coordinates". You might as well try to reflow a hardcover book to fit into your pocket better.
