You can get the URL for a document in the links section of the document details page. So you could copy the link and send it to someone via email manually, however, if they don't have an Alfresco account to login with you'll need to add ?guest=true to the end of the URL you send and also make sure the document is accessible to the guest user.
If you want to automate the sending of emails you can setup rules on spaces and choose the send email action. Alternatively on a document itself you can use the "Run Action" menu and again select the send email action.
Hope this helps.
