http://www.yourStore.com/PaypalPDTHandler.aspx is for versions 1.X http://www.yourStore.com/Plugins/PaymentPayPalStandard/PDTHandler is for versions 2.X
Hover over the submenu item [PROFILE] and choose the "My Selling Tools" option.
You will see the following section: Website Payment Preferences Auto Return for Website Payments Auto Return for Website Payments brings your buyers back to your website immediately after payment completion. Auto Return applies to PayPal Website Payments, including Buy Now, Donations, Subscriptions, and Shopping Cart. Auto Return:
When a customer makes a payment to you or a payment is reversed or refunded, PayPal will post a notification to your server at the URL you specified. Included in this notification will be all of your customer’s payment information (e.g. customer name, payment amount) as well as a piece of encrypted code. When your server receives a notification, it will then post the information, including the encrypted code, back to a secure PayPal URL. PayPal will authenticate the transaction and send confirmation of its validity back to your server.