I noticed this the other day when viewing a pdf of a bank statement. I assumed that my banking activities had been automatically "redacted" by Preview :0
Seriously, if you turn off “smooth text and line art” Preview will at least try to render the pdf as you zoom out.
—GetRealBro
Do Apple have any intention of fixing the inability of Preview to correctly render high resolution or vectorised PDFs/images when zoomed out (appearing blurry) or is this a new "feature"? It really does hinder design work...