I agree that PDF is best for things that are meant to be printed, no questions. But I wonder how common actually printing those papers is?

In research and in embedded hardware both, I've met some people who had entire stacks of papers printed out - research papers or datasheets or application notes - but also people who had 3 monitors and 64GB of RAM and all the papers open as browser tabs.

I'm far closer to the latter myself. Is this a "generational split" thing?

Possibly, but then again, when I need to study a paper, I print it, when I need just to skim it and use a result from it, it is more likely that I just read it on a screen (tablet/monitor). That is the difference for me.