I classify skimming as a "distracting" activity. I always ask myself: What information do I need? And then it's very easy to get to the relevant passage. And outlines are there for a reason (PDF Expert can edit them) so navigating between the same file is not that cumbersome for me. I do agree that the experience is more pleasant with a paper book, but in a focused session, the result is pretty much the same.

Skimming is great for building a mental map, but that is a separate activity than reading to learn. And it can be done digitally too, just differently.