Thanks to Yenthe you'll can study this precious little tutorial: http://www.odoo.yenthevg.com/inheriting-and-modifying-qweb-reports/

To edit PDF templates you can inherit template view of the PDF report and make necessary changes

I'm posting a sample code to inherit sale order template

<?xml version="1.0"?>
<odoo>
    <template id="report_saleorder_document_inherit" inherit_id="sale.report_saleorder_document">
       <xpath expr="//div[@name='payment_term']" position="after">
           <div>
              <strong>Some text</strong>
              <p t-field="doc.name"/>
           </div>
       </xpath>
   </template>
</odoo>

you may refer this link