Hi,

You can inherit the corresponding report template and remove the applied style.

Just activate the developer mode and navigate to Settings -> Technical -> Reporting -> Reports, search for Quotation / Order , click and open it, click on Qweb Views at the top right and open the template named report_saleorder_document.

See the highlighted section of the template which is responsible for the italic style. Just Edit and remove it from UI(which is not recommended) or create a new module inheriting this template and apply the changes.

For Reference: How To Inherit And Modify Existing PDF Reports in Odoo

Thanks