To make the linked RTE files forcefully downloaded by the browser, we need to add the following typoscript in the root template. 

lib.parseFunc_RTE.tags.link {

  typolink.jumpurl = 1

  typolink.jumpurl.secure = 1

  typolink.jumpurl.secure.mimeTypes = pdf=application/pdf, doc=application/msword, png=image/png, gif=image/gif, jpg=image/jpg

}