The format is designed for Kodak cameras to store uncompressed RAW images in their original form without auto color and white balance correction. The file has a TIFF-structure that allows you to edit the color temperature and exposure of the finished image without losing information and quality, which is something that cannot be done with JPEG format images.
This format was used for storing databases on the Pegasus and Palm PDA devices. Besides images, PDB allowed you to store tables, maps, e-books, and other data in a structured way. Today it has become obsolete and has been replaced by more flexible formats with DRM protection.