using (var client = new AmazonS3Client(tempCredentials,RegionEndpoint.USEast1))                        
    {
                    GetObjectRequest request = new GetObjectRequest();
                    request.BucketName = "bucketName" + @"/" + "foldername";
                    request.Key = "doc23.pdf";
                    GetObjectResponse response = client.GetObject(request);
                    response.WriteResponseStreamToFile("D:\\Docs\\doc234.pdf");//<---------
    }