A receipt is generated for every successful payment. There are two places to find them.
From the invoice
Open the paid invoice. The Pay button is replaced by a Receipt button — clicking it downloads the receipt PDF for the most recent payment on that invoice.
From the billing history
Go to Billing history in the left sidebar. Each row is a payment; click Download to get the receipt for that specific payment.
What if my invoice has multiple payments?
If you paid in installments or applied a partial payment, each payment has its own receipt. The invoice page's Receipt button always returns the most recent one; for older receipts, use the billing history page.