The .pdb file extension is a database format used for the Palm OS platform. The PalmDOC eBook file type is a version of the PDB format. The proliferation of eBooks has left this format behind due to the lack of support for DRM protection meaning only royalty free text is typically available.
The format supports reflowable content however the format lacks some of the features found in more modern eBook formats, for example only providing a very basic contents table. The format features RLE compression which was crucial because the format was designed for the low storage capacity Palm devices of the 1990s.