This one is easy :). Applications > Automator or just Command + Space and type in Automator
Build a Workflow
Drag the "Get Specified Finder Items" to the right hand side.
Now drag "Rename Finder Items" under "Get Specified Finder Items". You will be asked: "This action will change the names of the Finder items passed into it. Would you like to add a Copy Finder Items action so that the copies are changed and your originals are preserved?". I normally select "Don't Add" but if you want to be on a safe side select Add and it will create copies for you by default on Desktop.
Now all you have to do is select "Rename Text" from the drop down where you have "Add Date or Time" and select "extension only" instead of "full name". Specify what you want to find, for example "pdf" and replace with new extension, for example "pdfx".
Now drag some files into the "Get Specified Finder Items" and hit "Run".
Now you can save the Automator Workflow: File > Save.