Link to download pdf from webform email
If we are talking about a webform, then under the "webform" tab on the webform node there will be an "email" section. You can add it directly to the email text. The link will be /node/[nid]/webform/emails .

The "registration confirmation" part is a bit ambiguous. There is a user registration confirmation also, but I'm guessing that you are referring to the webform being a type of event registration form? In the case of user registration, there is a section under /admin/config/people/accounts where you can modify the emails.