During Sales Invoice creation, the Requester selects a Theme. After the Sales Invoice's approval, a PDF file will be generated based on this Theme.
Themes are configured in Xero and picked up by ApprovalMax.
If you want to change a Theme in Xero, go to Organisation settings > Invoice settings.
You can also upload your own DOCX template.
For more details please refer to Xero's Create an invoice template.