Hi, BookCat!
I understand that a PDB extension can actually be used for a number for files related to the Palm operating system.
However when speaking of ebooks, people are
usually referring to the eReader format. The software to read it on computers, PDAs, and some phones is available at eReader.com. (P.S. If you shop at Fictionwise.com and look at the "multi-format" books, you'll see they actually have
two PDB formats offered. One is eReader and the other is...uh, yikes, I can't remember the exact name...but it's an older format).
eReader formatted books - whether they have encryption (DRM) or not - are not natively readable on the Sony.
Some Sony owners use a Python script named "eReader2html" to ... um ... "liberate" their eReader books and convert them to other formats. I see you're in the UK - Harry, one of the moderators here, has made it pretty clear the using such scripts is considered illegal in the UK (as it is in the US, actually). Not the everyone chooses to follow that law, of course.
You still need to be aware of the legal issues, though, if you look into that option.
The only formats the Sony will read without any conversion are:
Sony's "BBeB" (which have the extension LRF for non-encrypted and LRX for encrypted), Adobe's version of encrypted ePub, Adobe PDFs (I think encrypted or not) - although PDFs have varying levels of success at being readable on the SOny - and RTF (no pictures will be displayed from RTF files).