In Invoice PDF it's not show serial number of product.  It will be used while processing delivery order. To show all serial numbers that is used in delivery order, you need to do customization in invoice report. You need to fetch those serial numebrs and show in invoice PDF report. 

Hi,

Try Following Sample Code

<odoo>
    <data>
        <template id="report_quotation_inherit_demo" inherit_id="sale.report_saleorder_document">
            <xpath expr="//table[@class='table o_main_table']//thead//tr/th[1]" position="after">
                <th class="text-right">Seari Number</th>
            </xpath>
            <xpath expr="//tbody[@class='sale_tbody']//tr//td[2]" position="after">
                <td><span t-field="line.your_field_name"/></td>
            </xpath>
        </template>
    </data>
</odoo>

Thanks

You need to customize an existing invoice pdf report to add your custom field. This link will be helpful.

https://www.odoo.com/documentation/11.0/reference/reports.html