I get where you're coming from but I'm not sure if PDF generation was a goal for mdbook to begin with. I'm pretty sure they're working on it now, but I'd stick to a domain specific tool, personally I find typst excellent to generate documents for prints.

For simple HTML docs mdbook is also excellent. I don't know if you could combine these two domains into one tool nicely. To me they're just too different.

although I have made presentations with mdbook using custom written preprocessors and a custom renderer (all of which were extremely crude, but did the job)