A point on one page in its default user space (ISO 32000-2 §8.3.2.3): where a viewport pixel falls on the page.
public PagePoint(int PageIndex, double X, double Y)
The zero-based page.
The x coordinate in default user space units.
The y coordinate in default user space units (y up).
ISO 32000-2 §8.3.2.3. Spec #33 story 54 (viewer capability 14).