APM Hosted files maximum size.

Host such a huge file in APM is definitely not a good idea.
You could host in a server publish with a new VS protected by APM and give access only to authorized users.

You could use multidomain sso feature to protect this new Virtual Server with the same policy you use for Portal. And publish also this new url as a link in Portal

So if anyone tries to reach this url without an APM session, will be redirect to your APM policy in order to authenticate.
After sucessfull authebtication he will be able to proceed with downloading.