Aggregation is the part-of relationship. It's like saying that a car has an engine and wheels as its parts. So IMOP, if your order only contain orderItem, it should be composition, otherwise you could use aggregation.
That's my 2 cents.
The Devil Wears Cartier Trinity
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com