ow can I ADD a document to be delivered via the DOWNLOAD form I just created?

You can attach your document to the autoresponder. So, your users will receive the document after they submit the form.

Please note that the supported formats are: PDF, DOC, DOCX, PNG, JPG, JPEG

You can check and follow this guide:

https://eu.jotform.com/help/393-How-to-Attach-a-File-to-Autoresponder-Emails

Let us know if you need any further assistance.