1. Go to Settings > Billing.
2. Scroll to Charges - Recent invoices and payment activity. Each row shows the invoice number, date, amount, description and a status badge (paid in green, other statuses in blue).
3. Click the download icon in the Receipt column; the invoice opens in a new tab from Stripe's hosted invoice page, where you can download the PDF.

The Plans table also has a Receipt link for the latest invoice of each subscription or add-on. If there are no invoices yet the section reads "No charges yet".