Beginning with PDF 1.6, the size of the default user space unit may be set with the UserUnit entry of the page dictionary. Acrobat 7.0 supports a maximum UserUnit value of 75,000, which gives a maximum page dimension of 15,000,000 inches (14,400 * 75,000 * 1 ⁄ 72). The minimum UserUnit value is 1.0 (the default).
15 million inches happens to be exactly 381 km Source
That’s sooooo arbitrary.
True. But thankfully most of the world uses centimetrs instead of inches nowadays. 😉