To add to sarah11918's comments about annotating pdfs, the annotations show up in the TOC as well, they just don't have a title. You can edit the TOC and give them a title if you want but just knowing I wrote something there is enough for me. I use the annotating exclusively to edit and comment on student papers. I send them back as pdfs with the annotation and they render beautifully on other machines and in color. I due use the blue "ink" and green "highlighter" so it stands out from the (usually) black type and the students can still print and highlight in other colors.