social/mail_as_letter/wizards/mail_compose_message.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2016 ACSONE SA/NV
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>
    <record id="mail_compose_message_form_view" model="ir.ui.view">
        <field name="name">mail.compose.message.form (in mail_as_letter)</field>
        <field name="model">mail.compose.message</field>
        <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
        <field name="arch" type="xml">
            <button name="send_mail_action" position="after">
                <field name="partner_count" attrs="{'invisible': True}"/>
                <button icon="fa-file-pdf-o"
                        type="object"
                        name="download_pdf"
                        string="Download Letter"
                        class="btn-default"
                        help="Save as a PDF file"
                        attrs="{'invisible': [('partner_count', '!=', 1)]}"/>
            </button>
        </field>
    </record>
</odoo>