How to display custom fields in PDF invoices

Version 1.5.3

I have created a custom field for invoices called PO. I went through the documentation on custom fields. But since I am not technical person, I could not understand much. Custom fields details are given below:

Table Label Type Order Position
Invoice PO Text 10 Custom Fields

Now I do not know how to show it on PDF invoices. Suppose the field value is AB/11.

I want to show PO: AB/11 on the pdf invoices. Please note that PO value will be different for each invoice. Can you please tell me which code do I need to add to which file?

I have replied to your other post…this is a duplicate.