To add the annotation click on the Annotation Toolbar to see the annotation tool icons.
By using this icons you can add different type of annotations While adding it will displayed in left side of the viewer panel.
By using edit and delete options 
you can add the comment and by using delete option you can delete added annotation. On clicking edit option you can add comment for the corresponding annotation using comment box .
After comment added in comment box click Reply button, your comment lines are automatically added in bottom of the annotation.
Now next user can response to the annotation added.
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