public class MyPdfViewer : PdfViewer
	{
        private ToolTip _tooltip = new ToolTip();

        protected override void OnMouseMove(MouseEventArgs e)
        {
            base.OnMouseMove(e);

            if (Document == null)
                return;

            int pageIndex = PointInPage(e.Location);
            if (pageIndex < 0)
                return;

            var page = Document.Pages[pageIndex];
            if (page.Annots == null)
                return;

            PointF pagePoint = ClientToPage(pageIndex, e.Location);

            foreach (var annot in page.Annots)
            {
                if (!(annot is PdfTextAnnotation))
                    continue;

                var rect = annot.Rectangle;
                if(PointInRect(pagePoint, rect))
                {
                    //any code showing annotation.
                    //Tooltip for example
                    _tooltip.Show(annot.Contents, this, e.Location);
                    return;
                }
            }
            _tooltip.Hide(this);
        }

        public bool PointInRect(PointF point, FS_RECTF rect)
        {
            return !(point.X < rect.left || point.X > rect.right || point.Y < rect.bottom || point.Y > rect.top);
        }
    }