I assume something like comments in Word, i.e. highlighting a fragment of document and "attaching" to it a piece of text. Not sure what is the good presentatio

I assume something like comments in Word, i.e. highlighting a fragment of document and "attaching" to it a piece of text. Not sure what is the good presentation for it: clicking or hovering, overlaying or pop-up or a separate window... I suppose to figure the best UX is not trivial, but even mediocre UX would be really great. And as others say, ideally keep the annotations outside of the document itself.