If some document requires an action from User, you can Assign that document to that User. On assignment, User to whom document is assigned is intimated via email. To learn about Assign To feature, [click here.]({{docs_base_url}}//user/manual/en/using-erpnext/assignment.html)
You can share document with the specific User. If Document is shared with the specific User, he/she will be able to access it, even if that User doesn't have permission to access that document or Document Type.