A family of Microsoft word processing software products for creating web, email, and print documents.
You will find a macro to do this at http://www.gmayor.com/word_vba_examples.htm. However for purchase orders, unless you are printing for filling them in with a pen, it would be better to create new numbered documents - http://www.gmayor.com/automatic_numbering_documents.htm and thus save your numbered orders for future reference.