Maybe you can try to define the font in your header most reports use the same header i think

<t t-name="report.external_layout">
<style>
    body {font-family:arial;};
</style>  
    <!-- Multicompany -->
    <t t-if="not o and doc">
        <t t-set="o" t-value="doc"/>
    </t>
    <t t-if="o and 'company_id' in o">
        <t t-set="company" t-value="o.company_id"/>
    </t>
    <t t-if="not o or not 'company_id' in o">
        <t t-set="company" t-value="res_company"/>
    </t>

    <t t-call="report.external_layout_header"/>
    <t t-raw="0"/>
    <t t-call="report.external_layout_footer"/>
</t>