private Rect GetControlDimensions(PdfControl ctrl, int pageIndex)
{
    Point pt1 = PageToClient(pageIndex, new Point(ctrl.BoundRect.left, ctrl.BoundRect.top));
    Point pt2 = PageToClient(pageIndex, new Point(ctrl.BoundRect.right, ctrl.BoundRect.bottom));

    //The page may be rotated, so we must take it into account
    int left = (int)Math.Min(pt1.X, pt2.X);
    int top = (int)Math.Min(pt1.Y, pt2.Y);
    int right = (int)Math.Max(pt1.X, pt2.X);
    int bottom = (int)Math.Max(pt1.Y, pt2.Y);

    return new Rect(left, top, right - left, bottom - top);
}

private bool IsFieldVisible(PdfField field)
{
   PdfControl ctrl = field.Controls.FirstOrDefault();

    if (ctrl != null)
    {
        Rect dimensions = GetControlDimensions(ctrl, CurrentIndex);

        if (dimensions.Top > 0 && dimensions.Bottom < ViewportHeight)
        {
            return true;
        }
    }

    return false;
}