The PDB file format is a compound file. It is composed of numbered data streams, each of which stores some predefined information about the target program. For example, one stream appears to contain a list of symbol names, while another describes the code and data sections used by the linker.
A special stream, the root stream, contains all the information necessary to locate the other streams within the file.
All numbers are encoded in little-endian format.
Credit to http://www.informit.com/articles/article.aspx?p=22685 for reverse-engineering an older version of the PDB format, which helped considerably.