The .pdb extension indicates a "container" file format that is used by many otherwise incompatible applications. FBReader makes it easy by only showing you the files it supports.

Under Linux usually the file command would tell you what kind of files you have, but it isn't on the 770. The Palm magic file for the file command is at:
http://opengrok.creo.hu/dragonfly/xr...ic/Magdir/palm

Check bytes 61-68 of your file directly to determine the format. FBReader supports:

Aportis Doc (bytes 61-68 = "TEXtREAd")
zTxt ("zTXTGPlm")
plucker ("DataPlkr")
mobipocket ("BOOKMOBI")

Some MobiPocket books show up as AportisDoc, and usually have a .prc extension. Note that DRMed MobiPocket books are not supported.