Microsoft published an official free program through which to download Windows 8.1 ISO.
You need download Windows Installation Media Creation Tool
http://windows.microsoft.com/it-it/windows-8/create-reset-refresh-media
Download link:
https://turbolab.it/scarica/32
Take attention, you can download ISO only if you have a valid serial.
If later you would like to create Live USB you can utilize this utility:
Rufus
https://turbolab.it/scarica/19
[original article https://turbolab.it/windows-10/scaricare-windows-8.1-dvd-iso-italiano-download-diretto-ufficiale-626]
