before earlier version 14, the download button in items table is shown even after the document is submitted, but in the most recent version 14(v14.22.1) this button is hidden, we can create the following client script to show this button again.
frappe.ui.form.on('Sales Order', {
refresh(frm) {
const wrapper = frm.get_field("items").grid.wrapper;
wrapper.on('change', function(){
wrapper.find('.grid-footer').toggle(true);
wrapper.find('.grid-upload').toggle(frm.doc.docstatus === 0);
})
}
})