The formal qualities of annotations are defined by the tools used to make them, and also the cultural uses of these. Given a pen, most people will write, although it is also possible to draw or use it in other ways. With a keyboard it seems most appropriate to type. With a mouse, one can click on things, and select them.
What happens when these are retained, transformed and isolated from the source?
These are rare, but I found one PDF of Jorge Luis Borges "The Garden of Forking Paths" that had underlinings and highlighted text. The interesting thing is that these change appearance slightly depending on the e-reader software used to display the text.
Digital annotations on a pdf of Jorge Luis Borges' "The Garden of Forking Paths"
Another text that came pre-annotated was Ursula K Le Guin's "The Carrier Bag Theory of Fiction". For this, I placed scans in a vector graphics program and digitised pen marks. I also included the text that was annotated with circles, underlines and lines in the margins.
Hand-drawn annotations on a pdf of Ursula K. Le Guin's "The Carrier Bag Theory of Fiction"