Gets or sets the part of the page to render, in default user space (the coordinates of the page's crop box), or
null (the default) for the whole page.
public PdfRectangle? Region { get; set; }
Spec #33 story 14 (viewer capability 1). With a region, GetPixelSize(PdfPage) is the size of the pixels the region covers at Scale and Rotation (rounded outwards to whole pixels and cut to the page), and GetDeviceTransform(PdfPage) maps the page so that those pixels are the bitmap's: each one is the pixel at the same place in a render of the whole page, which GetPixelRectangle(PdfPage) locates. Paints entirely outside the region are skipped (Replay(IRenderingBackend, PdfRectangle)), so a small region of a complex page renders quickly at any scale.
ISO 32000-2 §8.3.2.3 (default user space) and §14.11.2 (the crop box is what is shown; a region outside it shows nothing).