How to populate custom field into invoice

Guys i added new field into Purchase Order (PurchaseBy) how to populate into invoice (PDF ).

So you want to add that new field to the invoice report? If so you need to modify the PurchInvoice report (AX 2009).
This report has the invoice related tables in the query. If you have added this field to purchTable, you can get the purchTable by using \Data Dictionary\Tables\VendInvoiceJour\Methods\purchTable.
See how the data from other tables is being added to the report(for your reference)