FormHighlightColor = Color.FromArgb(25, 255, 0, 0);

public class MyPdfViewer : PdfViewer
{
    protected override void OnInitialized(EventArgs e)
    {
        base.OnInitialized(e);

        FormHighlightColor = Color.FromArgb(25, 255, 0, 0);
    }

    internal static int ToArgb(Color color)
    {
        return (color.A << 24) | (color.R << 16) | (color.G << 8) | color.B;
    }

    private FS_RECTF _invalidateRect = default(FS_RECTF);
    protected override void OnFormsInvalidate(InvalidatePageEventArgs e)
    {
        base.OnFormsInvalidate(e);
        _invalidateRect = e.Rect;
    }

    protected override void DrawCurrentPageHighlight(DrawingContext drawingContext, int pageIndex, Rect actualRect)
    {
        base.DrawCurrentPageHighlight(drawingContext, pageIndex, actualRect);

        var ptTopLeft = PageToClient(pageIndex, new Point(_invalidateRect.left, _invalidateRect.top));
        var ptRightBottom = PageToClient(pageIndex, new Point(_invalidateRect.right, _invalidateRect.bottom));
        Rect rect = new Rect(ptTopLeft, ptRightBottom);
        drawingContext.DrawRectangle(new SolidColorBrush(Color.FromArgb(30, 0, 255,0)), null, rect);
    }
}