In case you are still looking for the specs, you can find it here:
Note also that the Acrobat 9 SDK includes now a 3D SDK to read/write PRC.
Hope this helps.
IMHO, it'd be logical to put the specs or a link to them at
next to the link to U3D specs.
It is worth mentioning that that one more version of the spec can be found
in an ISO Working Draft of PRC format at
It seems to be more complete than the html docs in Acrobat SDK, really welcome are some additional explanations of what
the parameters really are for, where SDK version has just their names and types.