From Kevin M. on the users mailing list:
A confidential file doesn't display its image because the objid is unaccessible, due to a malformed xref table:
The first number (1) means the number of the first object. So it would be 1. The second number(7) is the size of the table. The number 1 is incorrect, it should be 0, because "0000000000 65535 f" is the dummy object 0.
From the pdf spec:
The free entries in the cross-reference table form a linked list, with each free entry containing the object number of the next. The first entry in the table (object number 0) is always free and has a generation number of 65,535; it is the head of the linked list of free objects
The error messages are:
Can't find the object 7 0 (origin offset 359138)
Missing XObject: Im1 (Im1 is the object at 7 0, and is the missing image)