Found this recently:

http://mpickering.homeip.net/phpwiki/index.php/DocReader

this links to DocReader, a freeware app that'll let you read &
create plain .pdb (PalmDOC) files. It's not fancy, but you can
proof your files before you're done with them.

I've used it for a little while now, and it seems reliable enough.

It also works great at converting pdb to txt for ebook readers like my Sony PRS 505.