public class MyPdfViewer : PdfViewer
{
    private Dictionary<int, int> _mySelecedPages = new Dictionary<int, int>();
    private Pen _pen = new Pen(Brushes.Red, 4);

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

        if (!ModifierKeys.HasFlag(Keys.Control))
            _mySelecedPages.Clear();

        int pageIdx = this.PointInPage(e.Location);
        if (pageIdx >= 0)
            _mySelecedPages[pageIdx] = 1;
        else
            _mySelecedPages.Clear();

        Invalidate();
    }

    protected override void DrawCurrentPageHighlight(Graphics graphics, int pageIndex, Rectangle actualRect)
    {
        if (ShowCurrentPageHighlight && _mySelecedPages.ContainsKey(pageIndex))
            graphics.DrawRectangle(_pen, actualRect);
            
    }
}