Earlier quoted context omitted.
Adapting PDFs for devices won't save us. HTML, being designed around reflow, had the ultimate solution from day one - and yet we've managed to screw that up so badly it spawned a whole industry sub-specialty of "responsive design". When authors start producing multiple PDF versions for different devices and print, how long until someone gets tired of "extra work" and comes up with "responsive PDFs"? (Also I feel that…
Like you mention, HTML already exists for adaptive text reflow. I assume that people making PDFs want their layouts fixed. But maybe an A5 format would make more sense, even if you're printing it? Also: What did people screw up with HTML in your opinion?
The problem with PDFs is that you need to create multiple layouts to make them look good in print and on a variety of commonly used screen sizes; all those layouts is extra work. HTML, by its very nature, doesn't have this problem, and yet somehow today we still have to design multiple layouts to support print and common screen sizes. And in practice, we usually don't - instead, we design one layout optimized for mobile phones, and ignore how bad lit looks on desktop or in print. "Responsive web design" turned into forcing HTML to behave like a PDF, except using "iPhone" instead of "A4" as the size.