Python can be used on Window, Linux and Mac. It just a simple install.
#!/usr/bin/python
import string
import os
import re
# task:
# loop thur input directory
# look for .pdf
# replace space with underscore
# rename/move to output directory
inputDir = "./" # Current directory
outputDir = "./new_location"
for filename in os.listdir(inputDir):
if filename[-4:] == ".pdf":
tempName = filename
tempName = tempName.replace(' ', '_')
os.rename(inputDir + "/" + filename, outputDir + "/" + tempName)