Gets or sets a value indicating whether a page render starts from transparent black instead of opaque white, so that the
pixels nothing paints keep zero alpha and partly covered ones are partly transparent; false by default.
public bool TransparentBackground { get; set; }
ISO 32000-2 §11.4.7: the page group is composited over a transparent backdrop, (Cg, αg), and the second step that composites it with the medium's white, C = (1 − αg) × W + αg × Cg, is left to whoever shows the bitmap (premultiplied BGRA). An opaque render is that step done over white.