I believe this is called "responsive design", a useful but differentiated concept.
It's pretty cool how PDFs don't typically load resources from an external website or CDN. They are self-contained, and thus demonstrably portable.
Gross analogy: Have you ever been to a multi-day music festival? A port-o-potty in the dark is disgusting, but still somewhat useable compared the alternative of going to the bathroom in public. PDF artifacts are in the same ballpark.
Also, I think I'd personally dislike a "portable document format" that doesn't look identical on all systems. By making it visually identical everywhere, you of course sacrifice comfort on smaller screens by nature. But to me that's an acceptable tradeoff for knowing I'm looking at the document, as it was intended to be displayed. I don't think of PDFs the way I think of websites. My physical papers don't reflow text.
It's pretty cool how PDFs don't typically load resources from an external website or CDN. They are self-contained, and thus demonstrably portable.
Gross analogy: Have you ever been to a multi-day music festival? A port-o-potty in the dark is disgusting, but still somewhat useable compared the alternative of going to the bathroom in public. PDF artifacts are in the same ballpark.