There are like a thousand of ways to do this, you should try google too.
The shortest way (to explain) is using bash like this:
Code:
for FILE in $(ls PD_*fr*mp3); do mv $FILE "${FILE//_fr}" ; done
Code:
for FILE in $(ls [0-9]*mp3); do mv $FILE PD_"${FILE//_fr}" ; done
The first for, is for files starting with PD, containing fr and ending with mp3. It just changes their names to a name without the _fr part.
The second one moves any file starting with a number and ending mp3, to PD_"+its name". So no files starting with PD_ will be changed. But any already existing files would be overwritten. Might want to try "mv -i" just in case.