whats the easiest way to add a field on checkout that allows customer to attach a document to the order

Do you mean the email message that goes out to the customer that has their Order PDF attached, and you want to let the customer to add something to both the Sales Order record that gets created on finishing checkout, and to the PDF?

no simply when they place an order on the website. I want to have a way for them to attach a file to the sales order that they are placing online

You mean so that it shows up in the Files sublist on the Sales Order record in the NetSuite UI?

@Livio this will require custom file upload extension and map it on a custom file field.