Guess what, I sometimes read PDF files in GoodReader on the tiny 3.5-inch screen of my iPhone!

Both GoodReader and the Retina screen are so superb that they make it possible. It's definitely punishing on the eyes, though.
I try to reduce the eye-strain wherever possible -- opting, therefore, for displaying comfortably large font sizes in EPUB files, too. So while it's perfectly possible to read PDF files on an iPad mini, the eye-strain would definitely increase compared to iPad Air; it would no longer be as
comfortable.
(I can judge this based on my Nexus 7, where the situation gets even worse due to the 10:16 aspect ratio -- particularly unsuitable for PDF files.)