I am legally obliged to distribute a document (probably by email, probably saved as MS word, or a PDF) to several hundred recipients.

The recipients are legally obliged to keep it confidential. However, based on past experience I'm pretty sure it's going to end up publicly leaked pretty quickly. (in the past it's been freely distributed verbatim)

This has happened before, it's a serious problem and causes us financial damage and I'd really like to put a stop to this and identify the miscreant.

I'm aware of the John Le Carre technique of making each document very slightly different (missing full stop here, minor typo there etc etc) but with several hundred recipients making several hundred uniquely identifiable copies of the same basic document would be a non-trivial task.

Is there a way to automate this? or is there a better way of finding who's doing the leaking?

UPDATES - documents published 2 or 3 times a year. In the past the whole pdf has been published verbatim on public or semi-public forums, often within days (sometimes hours) of being distributed. On other occasions the documents have been re-distributed via email from 'burner' accounts (normally gmail)