Amiga games are free to download on the Internet because it's easy to make them available. Most are still under copyright, but
a) No-one cares too much - it's only when big companies like Nintendo steps in that websites get shut down.
b) Some Amiga games have been made legal to download, but often only on certain websites.
So, essentially, it is technically illegal to download most of them.
HD disks can probably be formatted as normal low density on an Amiga, but I wouldn't put too much trust in the disks. It's better to use a HD disk drive on the Amiga for HD disks.