Ledger easily handles accounts receivable/payable, though it doesn't actually generate the invoices for you. Personally, I use org-mode for time tracking and in

Ledger easily handles accounts receivable/payable, though it doesn't actually generate the invoices for you. Personally, I use org-mode for time tracking and invoice generation (invoices aren't even PDFs ATM, though I have had clients that insisted on them), then I add a corresponding transaction in a ledger file when I send the invoice or receive a payment.