The downgrade from iOS 11 to iOS 10.3.3 has been stopped by Apple

As happens every time, two or three weeks after the release of a final version of iOS, Apple deletes the signature for old versions of iOS, that they can no longer be installed on iPhone, iPad or iPod touch. Not the same thing happens with Mac, where an older version of macOS / OS X can be reinstalled at any time.

If you have upgraded from iOS 10.3.3 or from an older version to iOS 11, we have bad news. If you want to go back to the older version, there is no way to do this. A short time ago, Apple deleted from its servers the signature that allowed the installation of iOS version 10.3.3 with the help of iTunes.
So you even have the IPSW files of the old operating system, they will not help anymore.

If you currently have an older version of iOS on your iPhone, iPad or iPod and you don't want to upgrade to iOS 11, make sure you don't start the update by mistake. Once the update process has started, you no longer have a chance to install the old version of iOS.

Currently, the latest version of the software is iOS 11.0.2, and the only one downgrade option it is at iOS 11.0.1.
Soon, Apple will release iOS 11.1, which is currently available in beta through the Apple Beta Software Program.