Create report in WHMCS with PDF export

We use WHMCS as our billing software. We want to create a letter for customers with outstanding invoices. The letter will contain: - Customer details (name, address etc) - Open invoices (invoice number, date, expiry date, total, already paid (if applicable), total invoice due and total all invoices due. An example letter is attached. It is Dutch but it should give a clear idea of what needs to be done. Requirements: - We need to be able to add the base document (as attached) - Report should have a selection field to select how many days the invoice is overdue. For example: == Enter number of days overdue: [textfield] Customer 1 | Total overdue E 50 | download PDF Customer 2 | Total overdue E 55.50 | download PDF ==