Good question jleine. However, only responses to questions can be marked as 'helpful.' The thinking on this is that a question has an answer, yet other responses can be helpful to that answer. Typically a doc is generally posted independently to provide insight on a topic. A doc could be posted in response to a question - but then it would be an attachment to the response.
I agree, tho - a doc can be helpful and it would be nice if the system allowed for noting that. An alternative option would be for you to add a comment to that doc noting how it was helpful to you. someone else may see your comment with the same issue.
The origin of my question is that I have a "quest" called onboarding and it requires me to like a document, but it appears I cannot do it. I am assuming there is some terminology issue or misunderstanding