Cannot start the conversion job because it does not contain any files.

Scenario:
While working with a small POC. I encountered a weird error.
Cannot start the conversion job because it does not contain any files.

Solution:
Error was coming when I was trying to convert all files of a document library from word to pdf.

I googled the error - but no luck , so thought of posting once I found the fix.

I read error and as per the message there are no files in the library.
I validated and there were files in there. So what could be it :)

Permission.
I checked the account running the Word Automation Service and gave "Contribute" access the account. That was it.