There are two possible ways of including content's download within an embed code.
The first way is by setting the displayDownload parameter to true. This will enable a download section within the embed code; from this section users will be able to download all the linked content previously included via addLinkedContent web service, as illustrated in this article.
The second way is by including in your page a direct link which will trigger a multiple content download. Detailed instruction on how to execute this type of integration can be found in this article, code samples are provided.