1. Grab pdn2png
2. Drop the exe from the zip into a folder with all your .PDNs
3. Open a command window, navigate to said folder, and run:

forfiles /m *.pdn /c "cmd /c pdn2png @file"

Different command-line switches can be used to change output options, but this worked for me to quickly convert a folder of 128 2-layer PDNs to PNGs.