Try it with Chrome.
When I clicked on the Download button on itch.io it took me to another
website, alterVista, where I was able to download the file.
I just want to verify that this is what you were expecting?
Itch.io Download:
Redirected to website:
http://hl.altervista.org/split.php?http://somebitsofme.altervista.org/misc/zx81/...