Downloading high sierra os produces corrupt file suggestions

Delete any copy of "Install macOS High Sierra" from the Applications folder. Try again while in Safe Mode.

How to use safe mode on your Mac - Apple Support

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS High Sierra App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS High Sierra.app in Applications then a new copy more recent version can be downloaded.