Change pdf field names

I would like to modify the pdf for the invoice, instead of showing item tax, it should show overdue charges, where can I change this in the code?!

Please search the wiki before posting:

My bad. Thanks

Under what file can I find this

stdClass Object
[invoice_custom_id] => 13
[invoice_id] => 24
[invoice_custom_archive_id] =>
[client_custom_id] => 8