The .pdb format is a native Maya particle caching format that is exported via the MEL or Python dynExport
command. It's intended for people that want to export particle simulations out of Maya and render them via a third party renderer like Renderman. As stated in the Maya documentation
for the command, there is no formal file specification. Maya does ship with an example open source program that reads the .pdb format, so you can start there. The source files also include a README file which describes the format a little more.
I don't have a Maya installation handy, but in versions I've looked at before, the source is located in $MAYA_LOCATION/Gifts/readpdb, and the Maya 2010 documentation listed above seems to indicate the source is now in $MAYA_LOCATION/ExploreMe/particles/readpdb. In addition, since the Dynamica plug-in is open source, we did include the source code for writing out .pdb files, so between the two, you should have code to read and write .pdb files.
We wrote the plug-in with .pdb support because that's what we use at Disney, but we anticipated that others would add their own exporters for their own preferred formats. As such, we have no plans to release the Houdini SOP that reads .pdb files, since that has code that is specific to our workflow, but I can say that it was a simple wrapper around the readpdb code provided above, just wrapped up in Houdini HDK framework code.
Hope that helps!