About 1 in 15 of the pdfs I view in Preview have some sort of "corrupt jpeg" artifacting on the first page. (These are scans of old books, magazines, typically.) There will be a diagonal staircase of green blocks, each what I assume is the DCT 8x8 size going all the way down the page, along with what looks like a missing color channel. The same pdf if opened with Firefox will look fine (so pdfjs gets it right). This has gone on for years, something to do with MacOS's own rendering of pdfs (so trivially switching to another app is unlikely to fix it). I have no idea what this is, I can't even think of a good description with which to google the problem...

Love to see an example PDF. (I'd like to send it to the PDF/ImageIO team at Apple.)