how to make files in icloud offline

According to this article Get help with iCloud Drive - Apple Support

What if my files don't update to iCloud?

Check these things first:

• Make sure that you have enough iCloud storage. If you need more space in iCloud, you can buy more iCloud storage or free up space by deleting content.
• Sign in to iCloud on all of your devices and turn on iCloud Drive.
• Make sure that you have an active Internet connection on your Mac and on your iPhone, iPad, and iPod touch.

So internet connection is required to update it .

If you don’t see your files in any of those locations, make sure that you set up iCloud Drive on all of your devices. If you don't see the files from your Mac Desktop and Documents folder on iCloud.com, go to System Preferences > iCloud, then click Options next to iCloud Drive and make sure that it's turned on.

To make files in iCloud offline in iOS , click on settings then click on wifi turn the slider as off .

I think MHLtwe meant something else. To answer his request: files in the cloud are only copies of what you have on your devices which linked to iCloud and set up to store data in it. So the files are always offline and online at the same time. By swithing off WiFi you would only stop iCloud from synching files, which it would continue once you get back online again unless you delete or move the file(s) out of the locations from where it would synch.