Hi
I used all four methods to fetch the purchase invoice number in print format, some some reason it is not working. I searched .js, json and .py files to get the field and it seems it should purchase_invoice or bill_no (from JSON) , but it is not working
Below is the attached output
http://imgur.com/oqBdaZn
To show the invoice number which is the ID of the Sales invoice use:
{{ doc.name }}
Remember this is true for All Transaction IDs.
@adityaduggal Thanks, it worked. It should have occurred to me, I was being little dumb on this 
I searched all around for the invoice number field !
@amruthp This is something which I always miss even till today