Hei Danielle, the challenge is the technology used, ie, the file is a OneNote stored in One drive for Business. Usually you would give access individually to a file to certain user, perhaps you can try to access the file location and try and give access to an external. In future, when Teams can be accessed by externals, you can save the link in a shared Teams site and see if you can access to the file (although I would assume you still need to give individual access to that file).
