Do you have screenshots that can demonstrate this? I'm surprised to hear it, I'm fairly picky about visual quality but I've never noticed an issue with PDFs on MacOS or iOS. Never used a low PPI screen with them though.
https://apple.stackexchange.com/questions/295427/what-causes...
https://discussions.apple.com/thread/8531638
https://twitter.com/deivi_dragon/status/938663751158501376
https://www.reddit.com/r/apple/comments/72lxjx/macos_high_si...
It is less visible on a high PPI screen although still there and noticable if you know it exists. Unfortunately I know it exists, can see it and am bothered, so I stopped using my Macbook and left it to collect dust.
In Yosemite PDF rendering performance was crap for me in some high resolution documents for print, but other than that it has been flawless for as long as I've been using macOS.
I've only seen the issue shown in the Reddit post (which looks like some rendering lag) in Android actually.
Maybe Apple actually optimized rendering for hiDPI displays since probably that's where the majority of users are these days.
Many professors in our department have a MBP, and their LaTeX presentations look bad, just because macOS is bad. I notice it every single time, and sometimes (without me even saying, I just tolerate it, don't make a sound) they themselves do, too, asking themselves whether they've grown that so old or something.
I only have the leftovers of my girlfriend, 2015 MBA, as a macOS device. The PDFs look like crap on Preview and many other ".app"s I have tried. SumatraPDF running on Wine works properly though. Yeah, I'd say Preview simply does not work properly at this point. Shame, but also fun to watch from the Windows's side.
It's just what they are used to.
Aside from blurry PDFs, I also used to like font rendering in OS X and prefer it to Windows but unfortunately it also got screwed up with Mojave after they disabled subpixel rendering.