Looking at the linked image in the OP and looking at the Description for the Compress Images in PDF Documents action in Automator, it says:
This action will compress the images within the passed PDF documents.
Therefore, you need to pass it the fully qualified pathname of the PDF document to be processed. So, as the last line in the Run Shell Script action, you can use the echo
command, e.g.:
echo "/path/to/document.pdf"