You can add a document, a link to another page in the lesson, or a web link.
Adding one
Click into the page where you want it, type /, and pick from the menu. Typing after the slash filters the list.
Document uploads a file students can open.
Page links to another page of the same lesson.
Link is an ordinary web link.
A document attached to a lesson is there for students to read. It is not the same as a tutor knowledge base, which is what an AI tutor answers from.