You could say something like "You will be able to download as soon as PayPal completes your payment; usually this happens by the time you finish checkout and continue, but sometimes it takes longer. You will also receive an email with a link to your download page once payment is complete."
BTW, if the buyer continues past checkout before PayPal has submitted IPN to us confirming completed payment, then the buyer would see a generic thank-you page with some helpful information like this:
https://www.e-junkie.com/ecom/rp.php