So they've opened the document, scrolled to the bottom, read the password (the first thing they'll do as soon as they realise there are passwords in each document) and 'completed' the task...
Maybe, dependent on the size of the document, but possibly migrate it onto pages, or a lesson, or something that allows you to track opening separate parts of the document - User Experience will be horrible, but so is the requirement.
Unfortunately, the requirement you've been given appears to effectively rule out any kind of checking whether they have read the content, rather than simply opened it and scrolled through it.