When preparing the document, you will see an Add tab in the top right. Click on it and you will see an Add Attachment or Link button. Once clicked, you will see this screen:
Drop a file into the upload box or click on Use External Link and enter the URL you'd like to attach. Title the file or webpage and check Add to Library if you would like to have easy access to it for future docs. Decide whether you would like it to be something required for your recipient to view and then click the Add button in the bottom right.