You can run an Invoice List report, filter it to show last month, export, and send it to your auditor. I'll show you how to do it:
Go to Reports.
Enter the Invoice List on the search bar.
Filter the Report period to Last Month.
Hit Run report.
Click the Export icon.
Once done, you can manually email it to your auditor. You'll want to visit this article to learn more about emailing sales forms like invoices in QuickBooks Online: Email and customise sales form.
Another option is to save the invoices as PDF and send them to your auditor.
Here's how:
Go to Sales from the left menu.
Select Customers at the top, then the appropriate customer name.
Find and select the sales form you want to download to view the transaction.
Click Print or Preview.
On the upper right of the PDF preview screen, select the Download icon.
Browse to your desired location, then select Save.