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…
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.