There are two issues here: 1. PDFs are fully baked/static and made for one page format (the one original book was printed on). These will never work. 2. Ebook

There are two issues here: 1. PDFs are fully baked/static and made for one page format (the one original book was printed on). These will never work. 2. Ebook formats are basically webpages and they have CSS. There are defaults from the ebook reader app and but often there are css settings from the publisher of the book. Their css styles mash together leading quite often to bad results. They could be very responsive but i suspect publishers test only on Kindle/ebook readers. I am not sure why they just don't make one "master" style thats tested everywhere. But it's probably because they are not software companies. For them epub/mobi just might mean "black and white ebook release". Also mobi is probably most of the sales and it is pretty restricted design wise compared to epub. So basically blame the publishers.