public class MyPdfViewer : PdfViewer
{
	//List containing the indices of the selected pages
	private List<int> _selectedPages = new List<int>();

	//Overrides the mouse down behaviour
	protected override void OnMouseDown(MouseEventArgs e)
	{
		try
		{
			//Determines the page to the mouse coordinates
			int pageIndex = this.PointInPage(e.Location);
			if (pageIndex < 0)
			{
				//If click outside the page, but in the viewer, then deselect all pages
				_selectedPages.Clear();
				Invalidate();
				return;
			}

			//if click inside selected page then do nothing
			if (_selectedPages.Contains(pageIndex))
				return;

			//Add a new page into list of selected pages and invalidate the PdfViewer
			_selectedPages.Add(pageIndex);
			Invalidate();
		}
		finally
		{
			//Execute the base behaviour
			base.OnMouseDown(e);
		}
	}

	//Overrides the drawing of current page highlight
	protected override void DrawCurrentPageHighlight(Graphics graphics, int pageIndex, Rectangle actualRect)
	{
		if (_selectedPages.Contains(pageIndex))
		{
			//if pageIndex inside a list of selected page, then highlight the border
			using (var pen = new Pen(this.CurrentPageHighlightColor, 8))
			{
				actualRect.Inflate(0, 0);
				var sm = graphics.SmoothingMode;
				graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
				graphics.DrawRectangle(pen, actualRect);
				graphics.SmoothingMode = sm;
			}
		}
	}
}