1. On the document you want to comment, click on the side of the menu.
2. Click Comment.
3. Type your comment in the text field.
4. Click Add.

From the document details page:

1. Find the document you want to comment on from the document search. 
2. In the right side panel, click the icon in the Workflow section. 
3. Type your comment in the text field.
4. Click Add.

The comment is now visible on the document data in your task list and in the document workflow and history.

If you have the professional view active in Invoices and Spend plans, you can find the comment action from (More actions) > Add comment.

Note: To view the document history, click the arrow beside the comment icon in the Workflow section and click View history.