The Amazon S3 module allows the drupal local file system to be replaced with S3, where the files can be uploaded or downloaded from S3. I used the following modules to achieve this:
Download and install these modules and place SDK library at sites/all/libraries/awssdk.
Check 'Status report' for information about the installation of Amazon S3 and AWSSDK.
Here we go:
In admin/config/media/amazons3, give your default bucket name which you would have created in S3.
Enter your "Amazon web services key" and "Amazon web services secret key" which are located in your Account settings at http://aws.amazon.com in admin/config/media/awssdk.
Change the file system path to "s3://" and set the Default download method to 'Amazon Simple Storage Service' in admin/config/media/file-system.
Set the Upload destination to "Amazon S3" in the Field settings of your Content-type.