Printing PDF to screen bug

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I encounter a bug when printing PDF to screen on an updated existing order
(quote or invoice as well)
I get a data stream of what looks like PDF binary data.
To reproduce go to an order, select print pdf to screen.
Will print fine every time.

Next go to the same order anew and add an item update and print pdf to
screen.

I have encountered this since sql-ledger days and now am running a modded
version of 1.2.18

Any ideas on this?
Thanks