For rpad, use this syntax:<?xdofx:rpad(INVOICE_NUMBER,10)?>
For concatenation, use either of these examples for sytax:
<?xdofx:3||2?> or <?concat(' ' , NET_DATE, ' ', NAME)?> -- in this case I am assuming that NET_DATE AND NAME are fields in your xml file.
In your case, you would need this:
<?xdofx:rpad(concat(INVOICE_NUMBER, ' ' , NET_DATE, ' ', NAME), 10)?>
It would be better to get this padding and concatenation done via your query and thereby in your xml file rather than doing this in RTF if possible..