Use the Shopify Digital Downloads app to attach a PDF or TXT file directly to a product.

Upload the file to Content > Files , copy the link, and paste it into your product description or a "thank you" email. Could you clarify which part you need help with? Are you trying to verify your domain with a TXT record?

A valid passport, driver’s license, or national ID card.

Do you need a for a specific country's payment gateway?

Recent utility bills or bank statements in the business or owner's name.