This can be done without fancy tools. For the last decade, I've used blogging software (CommentPress) and now often use Google Docs, which allow for threaded commenting.
But more recently, I am leaning toward open reading journals, which is a more traditional approach.
"As pre-class reading assignments replace lectures in flipped courses and serve as the primary mechanism for information transfer, it is essential that students complete their assignments before class."
Annotation-enhanced readings help students do that.
frontiersin.org/articles/10.…