In paypal_express.php around line 205 add:
// BOF Super Download Store v2.3.x mod
if ( (tep_output_string_protected($response_array['PAYMENTSTATUS']) == "Completed") && (tep_output_string_protected($response_array['PENDINGREASON']) == "None") ) {
tep_db_query("update " . TABLE_ORDERS . " set orders_status = '15', last_modified = now() where orders_id = '" . $insert_id . "'");
tep_db_query("insert into " . TABLE_ORDERS_STATUS_HISTORY . " (orders_id, orders_status_id, date_added) values ('" . $insert_id . "', '15', now())");
}
// EOF Super Download Store v2.3.x mod

Now download is only allowed when PayPal payment has Payment Status Completed and there is no Pending Reason.

Cheers!

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