A PDF document may have a maximum size of 14,400 by 14,400 units in default user space, and a minimum of 3 x 3 units. At a default unit size of a point, or 1/72 of an inch, a PDF document may not be larger than 200 inches by 200 inches. See Annex C, “Implementation Limits,” in the ISO 32000 Reference, 1.7, page 649. Find this document on the web store of the International Standards Organization (ISO).
You can specify the size of the unit in default user space on a page-by-page basis. Using the PDF Library, this can be controlled via the UserUnit declaration.
Without the PDPageSetUserUnitSize declaration, the MediaBox will display as 150 x 150 inches in Acrobat. With thePDPageSetUserUnitSize declaration, the above code will produce a MediaBox that will be shown as 300 x 300 inches.