When you use Seller Admin > Send Free Download Link, you will see a field for Email Message. Whatever you enter in there would be sent with the free link instead of the product's usual Thank-you Email for purchases (if one is set up in the product's settings). This would also be necessary for sending a free link if the Product Thank-you Email is disabled in that item's settings, or you could just copy the free link URL generated when you click Submit, then paste that link into a personal reply to the recipient.
The custom Email Message you enter here would appear in the message after our standard greeting and link presentation, or you can tick Enable Templated Email to have your custom Email Message completely override that, in which case you'd need to include the template tag
[%download_link%] in your message where you want us to insert the link (ideally on a line by itself). This help page lists other template tags you can use: