Due to special ink used in its creation electronic methods used to copy or scan the map into a computer automatically fail; when the map is introduced to bright light it becomes temporarily transparent.
This is from an adventure published in 2005, by which point the idea of taking a readable picture of a document using ambient light was, I think, reasonably well established even among non-specialists.
OK, maybe this guy just wanted to write a fantasy quest with a magic map, but had to put it in an SF setting because that was what he was being paid for. But it seems to me that, while being able to copy the Key Document may close some plot doors, it opens others: instead of “the bad guys attack us so that they can steal the map”, the adventure can become “we are racing the bad guys to decode and follow the clues on the map”. And it means you don’t need to come up with bad excuses for basic physics not to work.
Or maybe I’m just feeling grouchy tonight.