> If I pause the download process, and I turn off the
> computer then turn it back on the next day, is the
> data downloaded still stored?
Turning off the computer is no different than just terminating httrack.

If you *cancel* the process and then terminate httrack or power off the
computer you'll be able to resume the process whenever you want, without
having to re-download what has already been downloaded.

If you just pause it without canceling, or just terminate the process while
it's working, I'm not sure, if I recall correctly at least most of the times
it occurred to me I was subsequently able to continue without problems; I
wouldn't be surprised if there could be problems in some situations, though.

So if possible cancel it cleanly; if you can't or prefer not to (for example
because the canceling process is taking a long time, which happens), try...
In these situations, when you're not sure what can happen, it's probably wise
to make a backup of the mirror before resuming it.