The POs' PDF template is located here:
With the current ATUM version is not possible to copy the template to the child theme's directory because, while testing this just now, I've found a little bug. I've just fixed it and will be available on the next version. So you can copy the file to:
And to edit it there. In that way, your changes won't be lost on future ATUM updates.
Alternatively, if you can't wait for the next version, there an advanced way to do this through an ATUM's custom filter (atum/load_view/reports/purchase-order-html) that should return the absolute path to your custom template.
I hope this helps.