purchase_order_template.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>

    <template
        id="report_purchaseorder_document"
        inherit_id="purchase.report_purchaseorder_document"
        priority="100"
    >
        <xpath
            expr="//h2[@t-if=&quot;o.state in ['draft', 'sent', 'to approve']&quot;]"
            position="replace"
        >
            <h2 t-if="o.state in ['draft', 'sent', 'to approve']">Purchase Order #<span
                    t-field="o.name"
                /></h2>
        </xpath>
    </template>

</odoo>