@natevw

To your first point, you're absolutely right. Thresholding yeilds far more than an 8:1 compression because PNG is far more able to crunch bilevel graphics vs. grayscale.

To your second point, you're describing the #JBIG lossy compressor for scanned documents and monochrome images, and yeah, that's super cursed. I'd be surprised if that's what ocrmypdf is doing, but it's possible? ¯\_(ツ)_/¯

@Screwtapello

The fax compression algorithms are very limited, designed for a time where RAM was scarce. They basically compressed a couple rows of pixels at a time, nothing more, AFAIK.

There is #JBIG, which is a LOSSY 1-bit monochrome image compression algorithm. Yes, it is exactly as cursed as that sounds, there have been many cases where numbers and figures were changed by JBIG because a 6 looked like an 8. Horrifying. XD

I'm not totally sure what happened in this example, because I realized after I posted this that the pdfimages utility is converting whatever the PDF stored the images as into PNGs, not just extracting any embedded PNGs it finds.