For which platform?

https://f-droid.org/en/packages/com.module.notelycompose.android/

Why do you need it to be a browser plugin, a program that embeds itself into the webbrowser?

Firefox does have PDF view and edit with section jumping, comments, and highlighting. What do you mean by “easier”? What’s easier than a sidebar table of contents, and a toolbar with highlighting and comment actions? I don’t see what could be “easier” than that.