Using fnCellRender before exporting to pdf

Hello. I'm trying to export a pdf from a DataTables, but I'm facing a problem.
It doesn't matter where I put the function, it doesn't work.

I'm trying to do it like this right now,with no success.

"oTableTools": {
                "aButtons": [  
                {
                    "sExtends":    "pdf",
                    "sButtonText": "Save",
                    'fnCellRender': function ( sValue, iColumn ) {
                            alert('asdas');
                              // Append the text 'TableTools' to column 5
                            if ( iColumn === 5 ) {
                                 return sValue +" TableTools";
                        }
                        return sValue;
                    }
                }]
  }

I'm new to DataTables and would appreciate any help you can give me.
Thank you.