One question: Why do people make PDFs in A4 format? Wouldn't it make better sense to start making them in A5 or A6, so that they could be better read on e-readers, phones, and on part of a computer screen (which is landscape oriented)?
(Also I feel that by default, non-book PDFs tend to show up in the US "Letter" size, which looks deceptively similar to A4, until you try to print 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.
As for responsive HTML, it's the responsibility of the designer to make it work if he/she is worth their salt. Like you say, HTML without CSS is already responsive. If businesses understood that there are a big segment of customers who will always use their computer and never their phone when it's time to make a purchase, perhaps they'd be better at it.
I hate to break it to you but, https://blog.developer.adobe.com/adobe-sensei-makes-responsi...
I would have thought that a PDF of a book would normally be made in the size of the physical book, which could be A4, but usually isn't (at least not when I look at my bookshelves).
Edit: Also, are there any advantages with large papers like A4/Letter for physical prints, except that you can fit more on a single page?
You must be from North America. In the rest of the world, it’s always A4. I encounter A4 PDFs fairly often, but don’t know how long it would be since I encountered Letter, but easily years.