You can download all of your previous invoices on the Billing & invoices page of your account.

Click your profile image in the bottom left-hand corner of the Project Dashboard. From the menu, select Billing & invoices.

Then, select Open Billing Dashboard:
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Scroll down to Invoice History. Here you'll see all of your previous payments. Click the download icon next to the payment to download the invoice.

In the new tab that opens, select Download invoice.