Automator service does not work from Finder
Here is a simple Automator Finder Service that will do what I understand you're trying to do. It will render PDF pages with 200 dpi as PNG images then invert colours in them.



You will notice that I left the Show this action when workflow runs turned on so you can specify the folder you want the PNG files to be in.

Here is a test 3-page PDF file and the output this service generates:



PNG files output: