Before working with headers, text, or form fields, you should understand the PDF coordinate system.
Space in a PDF file, also known as user space, is measured in PDF units. The PDF specification defines PDF units as 72 PDF units to 1 inch.
For example: an 8 1/2" x 11" piece of paper converts to 612 PDF units by 792 PDF units.
Origin and Positioning
The PDF specification places the origin (0, 0) of a PDF document in the lower-left corner, with y increasing upward and x increasing to the right.