PDB (Palm DataBase) is a data container format developed for the PalmOS. It does not specifiy any particular internal format.
In addition to a wide range of application-specific data files that have used the PDB container, there have been a number of different and incompatible ebook formats that have used it. The most popular are/were the Palm Doc (aka Aportis Doc) and the eReader (aka Peanut Reader) formats. Plucker and a few others have also used it while MobiPocket chose to use the other Palm file structure, PRC (Palm ResorCe). As I understand it, the only PDB-based ebook formats that Calibre supports are Palm Doc and eReader.
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