When a file is shared with an external user, it is possible for them to download a copy of it by following these steps:
1. Go to the email notification that the file has been shared with the external user, and click 'Go to File'.
2. Fill in the verification code, which you will be sent in a separate email.
3. Scroll down to the bottom of the document and click the square symbol with the downwards facing arrow on the right hand side of the screen. This will then give you the option to 'Download a Copy' of the document.