I have a folder full of pdf files. I'm trying to remove all the spaces from files name and replace them with underscores. Here's what I have so far:

```
import os, sys

folder = path to folder
FileList = os.listdir(folder)

for files in FileList:
    if ' ' in files:
        NewName = files.replace(" ", "_")
        os.rename(files, NewName)

```

When I run this script I get the following error:

```
WindowsError: [Error 2] The system cannot find the file specified

```

I'm guessing there is a pretty simple fix, but I've look all over and cannot find a solution for the life of me.

Thanks for the help!