When you need that the PDF show some information relevant to your customers on the orders you may inlcude a note to it on the device you should go to
Main menu > Settings
Settings > Sales orders
Orders > Show order notes
On the Box type in the text you want to reflect on the PDF then Done
You may include a note on the web version too
Dashboard > Settings > Sales orders > Orders
Show order notes
Type the text you need to show on the PDF from web then Save