Hi,
There is no option to do that. What you can do however is to create a new custom field of the table "order" and of the type "file" via the menu Display>Custom fields. You can keep the "frontend" option of the field turned off in order to not display it on the checkout
Then, on the order details page of the backend, you can edit the additional information area of each order to upload your file in that custom field. And when you change the status and turn on the notification to the user, there should be a link to the file you saved earlier in the custom field.