Epub would be better than PDF for the use case. AFAIK, you can't reflow text in PDF, so if the page size isn't exactly matched to the device size, all you can do is pan&zoom. Yuck.

Not necessarily, as a significant part of a reading experience is the reader.

For example, I'm bound to a specific PDF reader that has a very powerful annotations system. The last time I checked Epub readers, there was nothing that satisfied my requirements.

A couple of additional notes:

- the downside of reflowing is that you may not be able to take annotations that require absolute positioning

- for almost all the PDF books I've read (books; magazines are a different story, but that won't work with Epub anyway), a 3:2 10" tablet is enough; if the text is not large enough, good PDF readers can mass-crop the pages. Of course, there are ugly exceptions - books that have a different text positioning on odd and even pages (I hate them).

Which reader is that?

It's Xodo. It has a good range of annotations, they work well, and they're very immediate to use/switch. It also has good enough scroll/zoom capabilities (its locking functionality is a bit lacking, but it's still good enough).

I don't imply it's best suited for everybody, that's the reason why I didn't specify the name.