1. Identify the QWeb template used for the invoice report. In this case, it would be the template with the report_invoice_document name. You can locate the template in the Odoo source code or search for it in your Odoo custom modules.

2. Override the QWeb template to include the delivery reference

3. Create a new template extension to add the necessary code. Here an example:
   

odoo>

data>

template id="custom_invoice_document_inherit" inherit_id="account.report_invoice_document">

xpath expr="//template[@id='account.report_invoice_document']" position="replace">

template>

xpath expr="//td[@class='trt-data text-right']" position="after">

td class="trt-data">

Delivery Reference: o.picking_ids.mapped('name').join(', ')"/>

/td>

/xpath>

/template>

/xpath>

/template>

/data>

In the above example, the custom_invoice_document template extends the account.report_invoice_document template. It adds a new td element after the existing ones, displaying the delivery reference using the o.picking_ids field. The mapped('name').join(', ') expression retrieves the name field value from the picking_ids and joins them with a comma.

Make sure to include the XML file in your custom module's manifest file