Thank you for your request. Sorry but from your request is not clear for which platform you need it. Since that please share with us which platform do you use?
In case that you use GroupDocs.Annotation for .NET you can do that in next way: you should set “User rights” for this user. You can do that with this code: .AccessRights(Groupdocs.Common.AnnotationReviewerRights.CanView)
This code should be added to the GroupDocs.Annotation widget.
In case that you use GroupDocs.Annotation for Java you can do that in next way: use this code AccessRights.CAN_VIEW in the widget.
In case that you need it for cloud Api (you use GroupDocs cloud service and the annotation embeded via iframe to your webpage) you should do it in next way:
use “AddAnnotationCollaborator($userId, $fileId, $ReviewerInfo)” method from the Annotation class.
As for viewing document with annotations via Viewer - sorry but viewer will view only document and annotations will be ignored.