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
Some questions…
That’s sooooo arbitrary.
True. But thankfully most of the world uses centimetrs instead of inches nowadays. 😉
To satisfy the anything but metric postulate: 381 km originally are 15 million inches.
However, using some tricks, the size isn’t limited to that.
https://alexwlchan.net/2024/big-pdf/
Remark: The side length of the square correspods to 381 km, its area is 145161 km^2.
Good to see Germany won’t run out of PDF for their digital mapping efforts and need a costly replacement.