Follow these steps to get the official ISO file from the Microsoft website :
Go to the Microsoft Software Download page for Windows 8.1: download windows 8.1 disc image -iso file- - microsoft
Microsoft removed the official tool link, but the tool itself still works. Download the MediaCreationTool.exe for Windows 8.1 from a reputable tech source (like the Internet Archive or Microsoft’s old TechNet library). Run the tool, select "Create installation media for another PC," and choose your language, edition, and architecture. The tool will download the ISO file directly from Microsoft’s Akamai CDN. Follow these steps to get the official ISO