Is there any workflow that I can create to convert a word document into PDF after the approval workflow is finished.
I have not seen anyone create a workflow capable of converting word files to PDF out of the box, but i have seen third party solutions that enable workflow features like this in SharePoint.
There is a custom action for workflows capable of that using Word Automation Service.
You could try Kaboodle Renditions at http://www.kaboodlekonnect.com/renditions. The Converter module can be configured to create a PDF (or other format such as XPS, TIFF, ePub etc.) automatically when a major version of a Word document is published.
So if the final step in your approval workflow publishes a major version then the conversion just happens. The beauty is that the PDF can be saved anywhere i.e. it can be in the same web or site collection but it could equally be a in a library in a remote SharePoint farm or even SharePoint online. And you can send metadata as well.
I should add that I work for Kaboodle Software, but don't let that put you off.