I have experience with OpenText Brava Enterprise. I've had the chance to integrate their Viewer in our in-house document management system. This viewer opens up to 100 file formats and allows to add/remove/edit annotations on files and saves these annotations as a separate layer on top of the file. So next time you open the same file to view, you can load the annotations (via API) calls and place the layer of annotations on top.
I was hoping I would see the same thing with Telerik PDF Viewer (and I am aware this works only for PDF). To allow the user to add a Text comment, Shapes, Stamps, Image, PDF Bookmark, etc.