Gets the bounds of what the node shows on the page, in page space (default user space, like Quad): its
glyphs' boxes, the paths and images its content paints, the rectangles of its annotations, or for a figure that paints nothing
measurable, its BBox attribute; null when none of these is known.
public PdfRectangle? Bounds { get; }
ISO 32000-2 §9.4.4, §8.5.3, §8.9.4, §12.5.2 (Rect) and §14.8.5.4.3 (BBox).