Digital Downloads with PayPal Standard is problematic.
First you have to understand that PayPal Payments standard does not process transactions in real time.
Just because you are on the checkout success page, does not mean that the transaction has been processed successfully. At that point, paypal has excepted the transaction and placed it in a queue to processed on a first come first served basis.
you can set your account up to use IPN to make a post to your page with the payment status, on the IPN page, you could set it up to send an email with the links of the down loadable files using Universal Email
see paypals documentation on IPN: