Another feature available in SharePoint Portal Server is the capability for users to subscribe to documents. Subscribing to a document means that you'll be updated every time a change is made to the document. This feature is very much like the Alerts feature available in SharePoint Team Services. To subscribe to a document, use the following steps:
Navigate to the SharePoint portal site.
Click the Document Library dashboard tab.
Click Subscribe to display the New Subscription tab.
Give the subscription a name. By default, the name of the subscription will be the name of the document; however, you can change this name.
Confirm the email address you want to use to subscribe to the document. You can enter a different email address from the one associated with your site membership if you choose.
Choose how often you want to be notified of changes.
Click OK to confirm your subscription.