PDF Coordinates

Using PDF Coordinates, the x-axis extends to the right from the page origin, and the y-axis extends upward from the page origin.

PDF units are used for width and height values (72 PDF units = 1 inch).
This means that the default 8.5 in. x 11 inch page is equal to 612 by 792 PDF units.
(Width (8.5 X 72) by height (11 X 72)).