In some PPT files, eg http://www.iisd.org/pdf/2004/climate_bus_maintenance.ppt, HSLF isn't finding a Document record, and hence can't load them via the model/usermodel interface. Need to figure out why PowerPoint has no problem with such files, and hence how to support them in HSLF.
I added a unit-test in r1613837 which loads the document and verifies that the slides can be loaded, so I think this was fixed in the meantime.