Hopefully some of the writers are reading this:

I love buying and reading physical books. However, about half of the books (I read mostly programming books) have letters that are printed pixelated. This is infuriating to me. No one bothers to run a trial print and see what comes out?

The root cause of this: PDF will look fine, but the text color is usually set slightly off black (why!!??). The eye couldn’t really see the difference and PDF renders smoothly. However, commercial printers couldn’t handle that properly.

Solution: set the text color to full black, you are using (most of the time) black and white printer!

You might need to have two PDF versions: one for printing and one for digital distribution (but why would you have off-black text anyway?).

> but the text color is usually set slightly off black (why!!??)

This can be cause by colour management. If the black is defined in terms of RGB and then converted to CMYK as part of the pre-press workflow, you'll typically have a mix of all four inks, and not necessarily 100% K - it depends on the colour profiles. For a black-only print job the C, M and Y channels will then be discarded, leaving a maybe-not-pure black.

This is why PDF/X exists

> why is the text color set slightly off black

Because pure black causes eye strain. Dark gray on white is superior for long reading sessions when your paper is white. The contrast really hurts after a while if you do pure black on pure white. This is a known phenomenon.

In fact, there's experimental evidence (https://www.nature.com/articles/s41598-018-28904-x) that this high contrast plays a hand in the onset of myopia, which in extreme forms is correlated with glaucoma and other vision disorders.