By secure im going to assume you mean if someone tries to steal the URL and pass it along to friends. Take a look at the GetOptimalURLs, this call lets you create a URL that will expire after a specified time period as well as locking it to a single IP Address. This should be sufficient to block almost all attempts to retrieve the file by anyone other than the intended user.
Let me know if you have any questions about using this call.
Regards,
Barry R.