The PDB extension does not specifiy a praticular ebook format. For that matter, it doesn't specifiy an ebook at all. It is merely a container type used originally under the PalmOS for data files (Palm DataBase).
There are quite a number of ebook formats that have used the PDB container structure. The two most popular were the eReader format and the PalmDOC/AportisDOC format. Apparently, your file is one of the many that Calibre doesn't support.
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