Hi Pia,

1- After activating the developer mode. Go to settings>Reports

2- Search for template name sale.report_saleorder > Click on Search associated QWeb views

3- Chose sale.report_saleorder_document.

4- Add your field to the report Architecture (here you must do some HTML coding)

If you have trouble to handle this, search for

[...]
<div t-if="doc.date_order" class="col-xs-3">                    
    <strong t-if="doc.state not in ['draft','sent']">Date Ordered:</strong>                    
    <strong t-if="doc.state in ['draft','sent']">Quotation Date:</strong>                    
    <p t-field="doc.date_order"/>                
</div>
[...]

past this lines after

<div t-if="doc.user_id.name" class="col-xs-3">                    
    <strong>Scheduled Delivery:</strong>                    
    <p t-field="doc.x_scheduled_date"/>                
</div>

Best regards.