social/mail_as_letter/report/mail_as_letter_qweb.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <record id="qweb_mail_pdf_export" model="ir.actions.report.xml">
      <field name="name">mail_as_letter</field>
      <field name="model">mail.compose.message</field>
      <field name="type">ir.actions.report.xml</field>
      <field name="report_name">mail_as_letter.report_mail_print</field>
      <field name="report_type">qweb-pdf</field>
      <field name="paperformat_id" eval="ref('report.paperformat_euro')"/>
      <field name="auto" eval="False"/>
    </record>
    
    <template id="mail_print">
      <t t-call="report.external_layout">
        <div class="page">
          <div class="oe_structure"/>
            <t t-foreach="o.partner_ids" t-as="p">
              <div class="col-xs-5 col-xs-offset-7">
                <address t-field="o.partner_ids"
                         t-field-options='{"widget": "contact", 
                                           "fields": ["address", "name"],
                                           "no_marker": true}' />
              </div>
            </t>
            <br/>
            <p>
              <b>Subject : <span t-field="o.subject" /></b>
            </p>
            <br/>
            <p>
              <span class="email_preview" t-raw="o.body" />
            </p>
        </div>
      </t>
    </template>
    
    <template id="report_mail_print">
      <t t-call="report.html_container">
        <t t-foreach="docs" t-as="o">
          <t t-call="mail_as_letter.mail_print"/>
        </t>
      </t>
    </template>
</odoo>