Pascal, I'm strugling with the same problem. Managed to change the font type of the HTML but as soon as you turn them back into PDF font is back to basic. 

Look at this one, it works for me today. It's the only CSS I need for the moment. 

<?xml version="1.0"?>
<t t-name="account.report_invoice_document">
    <t t-call="report.external_layout">
        <div style="font-family: Ubuntu;" class="page">
            <div class="row">
                <div class="col-xs-5 col-xs-offset-7">
                     <h4><strong>Invoicing address : </strong></h4>
                           <div style="margin-left: 20px;">
                                     <address t-field="o.partner_id" t-field-options="{&quot;widget&quot;: &quot;contact&quot;, &quot;fields&quot;: [&quot;address&quot;, &quot;name&quot;], &quot;no_marker&quot;: true}"/>
                                     <span t-field="o.partner_id.vat"/>
                            </div>
                </div>
            </div>

.......

See PDF with the margin-left and Ubuntu font.

Style should be defined inline. File containing custom CSS is not found during PDF conversion.