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>>The PK is not supposed to be visible or meaningful to the user. Not only that, but if you use order# + line # as the primary key in the line items table, you will have to do substring manipulations and/or cascade updates when the user changes the order #. This is even worse with a grand-child table. With a meaningless PK, there will be no need for such cascade updates.
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>In my app the user can NEVER change the order number, it is assigned by the application.

Still it has meaning to the users and compound keys will eventually get longer than any GUID.