They're not really comparable formats. PDFs are documents rendered onto virtual paper: there is only so much a mobile device can do there, as a PDF is likely rendered as an 8½ by 11 or A4, which isn't going to be great on a tiny mobile screen. Bad tradeoffs between tiny text or pan-and-scan.
eBooks, OTOH, are reflowable: ePub is "just" a ZIP of HTML¹. But the viewer can lay them out appropriately, so they can be formatted to fit your device, whether that's a tablet, a phone, or something else.
IMO if you want to read something on a device, ePub > PDF for those reasons. PDF is good for print, & ensuring that what comes out of the printer has a shot at resembling the screen. (Though there are a good number of print settings to mess that up, too.) And, as a passably portable document format, if you're just doing short term viewing or something.
¹Plus a lot of other metadata that I'm eliding; it's a fair bit more complicated, of course.
Ten years ago a “Tanya” told me she would never use an ebook to read literature. I’m pretty sure if I went to her house today I’d find 2. In fact I may be the only person I know who doesn’t own one (not wanting one and being fundamentally against them are two different things).
That’s the most memorable but I’ve had these sorts of “over my dead body” conversations on a hundred topics over the years.