Hope it be usefull to you

__init__

from . import "name of yor module"

__openerp__

{

'name': 'Name of your module',

'version': '0.1',

'depends': ['account'],

'author': "Daniel Hernández",

'website': 'http://www.odoohonduras.com',

'category': 'Invoicing',

'data': [

'views/report_account_invoice.xml', # In order to show your field in the invoice document

' "name of your module" _view.xml',

],

}

name_of_your_module.py

from openerp import models, fields, api, _

class account_invoice_line(models.Model):

_inherit = 'account.invoice.line'

your_field_name = fields.Char(string='Your field name')

your_module_view.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>

<data>

<record id="invoice_form_inherit" model="ir.ui.view">

<field name="name">account.invoice.inherit</field>

<field name="model">account.invoice</field>

<field name="inherit_id" ref="account.invoice_form"/>

<field name="arch" type="xml">

<xpath expr="//page[@string='Invoice Lines']/field[@name='invoice_line']/tree/field[@name='quantity']" position="before">

<field name="your field"/>

</xpath>

</field>

</record>

<!--<record id="invoice_supplier_form_inherit" model="ir.ui.view">

<field name="name">account.invoice.supplier.form.inherit</field>

<field name="model">account.invoice</field>

<field name="inherit_id" ref="account.invoice_supplier_form"/>

<field name="arch" type="xml">

<xpath expr="//tree[@string='Invoice lines']/field[@name='quantity']" position="before">

<field name="second_uom" attrs="{'readonly': [('state', '!=', 'draft')]}"/>

</xpath>

</field>

</record>-->

</data>

</openerp>

Abd in views folder

report_account_invoice,xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>

<data>

<template id="report_invoice_document_itemnumber"

inherit_id="account.report_invoice_document">

<xpath expr="//div[contains(@class, 'page')]/table[1]/thead/tr/th[1]" position="before">

<th>String of your field</th>

</xpath>

<xpath expr="//div[contains(@class, 'page')]/table[1]/tbody/tr/td[1]" position="before">

<td>

<span t-field="l.your field"/>

</td>

</xpath>

</template>

</data>

</openerp>

<xpath expr="//form/sheet/notebook/page[1]/field[@name='invoice_line_ids']/tree/field[@name='price_unit']" position="after">
        <field name="deduction"/>
</xpath>