Unable to Access MongoDB GridFS data with Spark using mongo-hadoop API

I saved a 2GB pdf file into MongoDB using GridFS. now i want process those GridFS collection data using Java Spark Mapreduce API. previously i have successfully processed mongoDB collections with Apache spark using Mongo-Hadoop connector. now i'm unable to GridFS collections with the following code.

MongoConfigUtil.setInputURI(config, "mongodb://localhost:27017/pdfbooks.fs.chunks" );
MongoConfigUtil.setOutputURI(config,"mongodb://localhost:27017/"+output );
JavaPairRDD<Object, BSONObject> mongoRDD = sc.newAPIHadoopRDD(config,
com.mongodb.hadoop.MongoInputFormat.class, Object.class,
BSONObject.class);
JavaRDD<String> words = mongoRDD.flatMap(new FlatMapFunction<Tuple2<Object,BSONObject>,
String>() {
@Override
public Iterable<String> call(Tuple2<Object, BSONObject> arg) {
System.out.println(arg._2.toString());
...
Please suggest/provide better API methods to access MongoDB GridFS data.