Kodak Photo CD support has almost gone, so the best and easy way to convert all PCD images to some more handy JPEG is to use the Photo CD Decoder pcdtojpeg. More information can be found on the project web site.
The terminal command will convert every *.PCD starting at current directory and subdirectories:
find . -name '*.PCD' -exec pcdtojpeg -q 100 -r 5 {} \;
Ah yes – and maybe your files have extension *.pcd:
find . -name '*.pcd' -exec pcdtojpeg -q 100 -r 5 {} \;
If pcdtojpeg is not installed global, use full path to where it is placed:
find . -name '*.PCD' -exec /path/to/pcdtojpeg -q 100 -r 5 {} \;