[Joel]

If they're read-only, I'd be willing to bet that they have more than one canvas, which the standard version of OmniGraffle 4 didn't support (it's a feature in version 5, however).

I'd suggest grabbing a copy of OmniGraffle Professional 4.2.3, and a 24 hour trial license so you can either do the presentation or split the file's canvases up into individual 1-canvas documents.