Inherit the pdf report template and add the above snippet of code to get invalid header in cancel state
<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="report_saleorder_document_inherited" inherit_id="sale.report_saleorder_document">
        <!-- Replace the stamp image with a red header text -->
        <xpath expr="//div[@t-if='doc.client_order_ref']" position="before">
            <div t-if="doc.state == 'cancel'" class="row mb-4">
                <div class="col-12">
                    <div style="background-color: #FF0000; color: white; text-align: center; padding: 10px; font-size: 24px; font-weight: bold; border-radius: 5px;">
                        INVALID QUOTATION
                    </div>
                </div>
            </div>
        </xpath>
    </template>
</odoo> 


Regards,
Nikhil
Accurates