Download apple bills from the apps I've bought

All purchases that generate an invoice would be sent to the associated email account. You should be able to get them all from there easily.

Otherwise, follow the steps in the support document you posted from and on an iOS device you an re-send the invoices to your associated email address by tapping on the "Total Billed" Line and then on Resend.

If logged in to the https://reportaproblem.apple.com website, you can download the invoice by clicking on the order number for the specific invoice at the top right corner of each purchase line, clicking on "View Invoice" and then right clicking it to save as a PDF.