In your qweb report template add css like below:

<style>  
    html *  
    {  
        font-size: 12;  
        font-family: Times and Roman;  
    }
</style>

Hope it will works

Salut regarde ce lien:

https://www.odoo.com/fr_FR/forum/aide-1/question/how-to-add-custom-fonts-to-qweb-reports-88923

Did you follow Odoo's advice and use wkthmltopdf 0.12.1 as described in Odoo's installation documentation? Does it happen with the proper version too?