If the PDF only confirmation of payment it might be too similar to a receipt. I'd instead send the user a digital assets "In exchange for the $X I'm sending you

If the PDF only confirmation of payment it might be too similar to a receipt. I'd instead send the user a digital assets "In exchange for the $X I'm sending you a hand-drawn limited edition picture of my cat". That said I know several companies that for years have used Paypal's donate button to run their whole company. It's seems arbitrary.