.pdb is NOT an eBook format. Never was. Never will be.
A PDB file is a "Palm Database". That's what .pdb stands for.
It can hold many, many different formats of eBooks. So you need to understand what kind of eBook it is.
If it's a PalmDOC, txt2pdbdoc will turn it back in to text without a problem. If it's one type of Mobipocket (an HTML file in a PalmDOC), it will work too.
However, you are pretty much out of luck if it's anything else.