'Download is unavailable' can be for two reasons.

1) You have used up all available downloads (once you have downloaded 3 times, no more downloads are possible.

2) There was a problem processing the uploaded data. This can be tied to system files in folder uploads (where the data might be hidden inside Windows).

If this is the case, the sender of the data will have been notified via email with some checks / troubleshooting tips to enable them to try again.