You can attach a document at Order level and same will be available when it is transferred to Invoice.
In order to do this, you need to attach the document for that particular Document Number but not at Order ID level.
This will work fine.
Just click the Note Icon next to Document Number when you start entering Order Transaction in SOP Window.
Click the Attachment Icon, and new window will be opened.
You need to click on attach so that you can browse through files in your system.
select a file and click ok.
now in Note Window, click attach.
You can see the Note Icon in Yellow color which shows that attachment exists.
When you transfer it remains for Invoice also.