I put it through a password complexity tester at http://howsecureismypassword.net/ and the result was "It would take a desktop PC about 802 vigintillion years to crack your password". (That seems to be a pretty long time).
That, of course, assumes a random password, which isn't likely the case here. There is almost certainly an algorithm creating these document ID's and if the algorithm can be guessed, that certainly ups the odds that someone can guess the ID's. But in and of itself, guessing the ID of a particular ID associated with a particular account would be exceedingly difficult. An attacker would need to be logged in as someone that has access to a specific document, which lowers the risk considerablly.