I refer to something like a 3000 page manual of some microcontroller, or the datasheets of some integrated circuits or the specifications of some Arm architecture variant, or the standards for some programming language, e.g. C++ or System Verilog.
These are concrete examples of documents that I might have read during some flights or when waiting for some flight, on a smartphone.
When reading something like a fiction novel, reflowing the text based on the window width may be acceptable.
On the other hand, the navigation through a huge document half of which are tables, figures, diagrams, schematics and graphics is extremely painful when it is in HTML format so the layout changes based on the device and window used and there are no means to jump quickly e.g. to page 1436, then to page 2117. When zoom, pan and scroll are correctly implemented, which unfortunately happens seldom, they are much less distracting than the random changes in page layout caused by rendering as done by a browser.
I strongly dislike whenever a company provides only a Web documentation that is hard to navigate, instead of also providing a PDF manual.
Web documentation may be acceptable for very small documents, but not for most of the current technical documentation, where many thousands of pages for a manual are common.
Perhaps an EPUB format extended with everything necessary to completely describe a fixed page layout might become competitive with PDF, but I will have to see an example to believe it.
For now, whenever I see a book or any other document both in PDF and in EPUB formats, I always choose the PDF variant, because without exception it provides a better quality of the rendered pages.