Program Database files are files that end with the extension .pdb and are associated with a library or executable (a .dll or .exe file). This format was until recently not documented so Mono used its own file format for storing debugging information, the Mono debugging format.
Just like PDB files Mono Debugging information is stored in files that reside side-by-side with a program executable or a library.
You can use the pdb2mdb tool to transform these PDB files into MDB files that Mono can then consume to debug programs or provide line-number information for stack traces or exceptions.