I’m wondering if this blurriness is due to html canvas stuff being used to render the pdf in the miniviewer - blurriness with 2dcanvas is not an uncommon issue. This article mentions some stuff with adjusting based on dpi - can you take a quick look in case it is useful for this issue? Fixing HTML5 2d Canvas Blur. Understand how to use DPI to fix Canvas… | by Zak Frisch | WDstack | Medium
or this: How to make the canvas not look like crap on retina (Example)