PDF does not have any capabilities of line breaking. It is a picture format, similar to SVG, only more rigid. That's why it can't have text reflow, etc.
What an ebook format needs is a semantic form of markup, which adapts to devices it is rendered on. HTML + CSS were invented for this goal.
With that, book layout authors should consciously relinquish some control on how the book looks, and hand it to the reader. Slight visual imperfections are a small price to pay for this. Who needs visual perfection should go for a PDF.
This, of course, becomes hard if any interactive stuff is involved. I would suggest that larger interactive elements should open in a dedicated view when needed, and tiny interactive elements should embrace reflow.
HTML (with SVG and MathML) is probably fine for most books, but CSS has spent 30 years resolutely resisting basic typography, i.e. default text baseline alignment.
What an ebook format needs is a semantic form of markup, which adapts to devices it is rendered on. HTML + CSS were invented for this goal.
With that, book layout authors should consciously relinquish some control on how the book looks, and hand it to the reader. Slight visual imperfections are a small price to pay for this. Who needs visual perfection should go for a PDF.
This, of course, becomes hard if any interactive stuff is involved. I would suggest that larger interactive elements should open in a dedicated view when needed, and tiny interactive elements should embrace reflow.