To go down a completely different route, has anyone investigated the open source javascript PDF viewer bundled in the likes of Firefox? This would be post-classilla, but as the code is available, maybe it works with a javascript runtime that'll run in OS 9? It might also be something that could be ported to C++ or MacPython.
This would have the added benefit that it supports modern PDF versions and just ignores the objects (encrypted forms, etc.) that it can't handle, instead of refusing to open the PDF at all.