To download original raw report that was ingested from PSP, you can make a request described on this page. The response will contain a download_url with a URL to go to to download the raw report.

To find id of the report to send as report_id in the request you can:

Check report_ids field on payout - we save all of the report ids that referred to particular payout.
Check report_id field on payout item - we save report id that produced particular payout item.
List all reports from the project to find the one.
Fetch single report by it's id to verify that it is the one.