Highly Compressed Windows 81 64 Bit Verified Link

Highly Compressed Windows 8.1 64 Bit (Verified): What You Need to Know

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Option 1: The Official Windows 8.1 ISO (The Best Way)

The Ultimate Guide to Highly Compressed Windows 8.1 64-bit (Verified): Myths, Methods, and Safe Installation

But what does "highly compressed" actually mean? Can a 4GB ISO file be shrunk down to 800MB without breaking core functionality? And most importantly, how do you verify that such a compressed file is safe, genuine, and bootable?