The amount of the total client overdues is not meant to be displayed on the template so you would have to modify the /application/helpers/pdf_helper.php to get the data and pass it to the template.
this may not be the approved way to get this to work, but i added the following:
IFnull((SELECT SUM(invoice_balance) FROM ip_invoice_amounts WHERE invoice_id IN (SELECT invoice_id FROM ip_invoices WHERE ip_invoices.client_id = ip_clients.client_id)), 0) AS client_invoice_balance
to the select statement in application/modules/invoices/models/mdl_invoices.php
and can access the value using: $invoice->client_invoice_balance