I guess you don't read on smaller device. Kindle, for example, with 6-7" screen, is not going to read A4-formatted PDF well at all. On phone, too. A4-sized PDF only works well on 10" device or larger.
That's a lot of condition to be perfectly readable.
EPUB is readable even without all those conditions because it's reflowable.
(And personally I hate using landscape mode on my phone/Kindle)