You can add custom fields to your invoices, to suit what extra information that you would like to have for your customers. To do this, go to the Settings cog > Accounts > Invoices.
Scroll down to Invoice PDF options, and Invoice Custom Fields. There, you can create a title for the new field, and the value that you would like displayed in the Invoice PDF. You can add more than one custom field by selecting + Add Another. To save the new fields remember to Save at the bottom of the page.
The new custom fields will show up under other labeled fields in the top section of the invoice.