You can include links, but not attachments. However, if you put the documents you want to share in a publicly accessible place on the internet (such as Google Drive or Dropbox) you can link to them in the email.
On the Promote page, there are "Edit Student Welcome Email" and "Edit Volunteer Welcome Email" buttons. Click the relevant button to edit the message that appears in the email and the "Thank You" page following registration. If you put full links (including the http:// or https://) in your message they will appear as clickable links.
Customer support service by UserEcho