Incidentally, I just looked into this. There's a document in the Plucker CVS that describes their file format internally.
http://www.tinbergen.net/DBFormat.html
There's also the PalmDOC format, which I think you want to know which is like
http://homepage.mac.com/pauljlucas/software/txt2pdbdoc/
Then there's also iSilo in pdb format and eReader in Pdb format.
.pdb simply means the Palm DataBase. A lot of applications use this -container- format. Contents, of course, differ.