What's a PDB file?

A Program Data Base file has nothing to do with incremental linking and Project State! PDB files are used to map EXE with SOURCES. They are used for Debug and Release binaries. Here an article that explains this binding link between an Executable Image and its PDB file

PDB files store the information that allows you to debug an application.

The reason that the files are usually not deployed is that there is usually no need to ship them. Should you want to debug the application, you can always load them from a directory, network share or symbol server.