Marginalia, then and now.

So, Okular works for PDF commenting, even better than Foxit. Annotating's a big deal for me, my books always have underlinings and scribbled comments. Like Heinlein said, once an editor pees on the work, then he likes the smell better.

I learned the trick from an old copy of Will Durant's The Story of Philosophy, found an old yellowing hardcover edition when I was a kid, used to belong to an aunt or a grandfather, had a lot of notes. It was fascinating, going through Durant's text itself, and seeing the marks and traces of a sharp mind that had gone before me.

Currently reading Transmetropolitan, and annotating my way through Oliver Sacks's The Man Who Mistook His Wife for a Hat.