All subscribers to a document or folder will automatically receive an email notification whenever that document is edited. Subscribers to a folder will automatically receive an email notification whenever any document in that folder is edited or added.
As the publisher of a document, you may also force the distribution of email notifications to that document's subscribers by navigating to the properties of a document and clicking on the Distribute button. This will send out an email notification to all subscribed users with a link to the most current version of the document.