This rigidity is actually the problem with PDF, it makes them unsuited for anything but printing. Trying to read a PDF that was intended to be printed in A4 format on a smartphone is an absolute pain. One of the best selling point of Epub is that you can change the font size etc… This is absolutely necessary to make ebooks readable on a small device. When it comes to websites with mainly text content, I almost always use firefox’s reader mode, which allows me to set the font size, as well as using a dark mode.

PDF gives absolutely zero control to the user on how they should be displayed.