There's a couple ways you could go about offering your ebook in multiple file formats. You could just combine them into a single .zip file and provide that file as your download product, or you could create a separate product for each file and then either sell them separately or create a third "bundle" product to sell, which would provide download links for both files together. See this help page for further details:
To provide your backers with free downloads, you could just use the "Send free download link" function in your Seller Admin to send each one a free link manually. When you send a free link, you can even specify a custom Email Message that would differ from the thank-you email your actual buyers would normally receive. You may want to send yourself a free link to test this first, so you can see how it looks and works.
However, if you prefer to have them actually order the product and apply a discount code to place their order for free, you would probably want to create a series of single-use discount codes, so each backer would get their own code to use. You would still need to send each backer their discount code, so this may wind up being at least as much work as just sending free links manually. We can followup with more detailed instructions if you prefer this approach, but meanwhile this help page explains our discount settings in detail: