If you are struggling with adding line numbers to dolibarr sales order or proposal or proforma invoice PDF, here are the magic:

Go to global configurations add the constant "PDF_ADD_POSITION", set value to 1.
Then try to generate pdf document and see the magic.

You can also set "MAIN_VIEW_LINE_NUMBER" to 1 to add line numbers on the detailed view of quote / sales order / proforma invoice.

The developer really likes to keep things secret!