The message templates on the system have link or add image options, but not attachment. If I want to send a document to new enrolled people, I would find an alternative way with current options.
Since this document is going to all newly enrolled learners, maybe it is best as a Course Resource or Object lesson object (in the case of an Online Course) so you can be sure that it was consumed?
We will keep an eye on this idea and reconsider if it garners more support but for now I would say there are other methods to achieve this (including Christina's excellent suggestion which would let an Admin track if the document is being accessed) with equivalent or more value delivered to your organization already in the LMS.
I do this by adding the item as a global resource then adding the link to open in a new tab, I know it isn't the easiest way to handle it, but it also allows the document to be posted somewhere that people can later access if needed.