Earlier quoted context omitted.
Annotations are super useful. Please consider storing the annotations external to the PDF. For example, Okular wants to save these to the PDF itself. This doesn't work very well with a read-only network drive (read: corporate environment).
What do you mean exactly by annotations? Sioyek currently does have a bookmark support which is external to the PDF (you can bookmark locations in PDF file and you can later quickly search in those bookmarks). But I assume you mean some sort of visual annotation that is drawn on top of the PDF?
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.