That’s not a PDF issue though: it’s just an issue of most content being the same dimensions as real world paper sizes. No idea how responsive PDF would work but I agree that would be cool.
People in these comments keep mentioning a responsive PDF, but PDF is supposed to be a presentation format. Wouldn’t HTML and CSS serve the purpose fine?
You can't represent vector graphics, complex math, or anything non-standard in HTML+CSS. Even if you could, you couldn't rely on the user seeing it correctly.