If you really get stuck I've got an old version of Photoshop (6.0) on my desktop. You could email the files to me and I could try to open/convert them for you. (Address on request).
However it occurs to me that GIMP (which I've also got on my computer) is designed to be able to do everything that Photoshop can (except make a hole in your wallet!) and that there might thus be a good chance that GIMP can open .pdd files. This seems to confirm that it can:
http://extension.nirsoft.net/pdd
(Unlike a simple file viewer, if GIMP can open a .pdd file it will also be able to save it, via File > Save As, into a different format, such as .jpg)
So GIMP could well be worth a try:
https://www.gimp.org/