Everything about Billing PDF Templates
@param type: possible values: INVOICE, ORDER, DELIVERYNOTE, OFFER, REMINDER
@param address_side: possible vales: left, right
@param bill_item_fields: order of fields in the template;
Ex. : qty,name,description,vat_percent,net_price,net_subtotal
Possible values:
currency
date
qty
list_price
discount_percent
discount_amount
net_price
gross_price
vat_percent
vat_amount
net_subtotal
gross_subtotal
name
description
@param is_default : bool; possible values: 1, 0 - only one template of a type can be set as default. the default template will be used in automatic processes
@param include_header : bool; possible values: 1, 0 - if the default address header should be used.
@param include_footer : bool; possible values: 1, 0 - if the default footer (with bank information) should be used.

For creating a totally customized document, a solution is to set include_header and include_footer to 0 and then to include the whole customized address inside the body.