I just spent 5 minutes trying this and have it working on my iPhone and iPad.
It really is straight forward, though more complicated on iOS than on Android, presumably because of Apple’s closed-off approach. (I don’t have an Android device, but if I were to run an Android Virtual Machine, I expect that opening an offline iGuide’s index.html file would be trivial.)
Here’s an overview of what I just did.
DOWNLOAD: Download the offline iGuide. Unzip it.
PICK A VIEWER APP: Since it sounds like iOS makes it complicated to open local files in a browser, you need to use an alternative to “view” the offline iGuide. Planitar has given you two suggestions already, I went with the free and excellent Readdle Documents app. See Documents: Files, PDF, Browser on the App Store.
TRANSFER: Next you need to transfer the offline iGuide files onto your iPhone or iPad. You could probably use iTunes, iCloud Drive, or other methods, but I used Documents’ very simple Wi-Fi Transfer feature to drag and drop the offline iGuide to my phone in a matter of seconds. See How to Transfer Pictures from iPhone to Computer | Documents.
VIEW: In Documents 5, open the index.html for that offline iGuide and you’ll have the full iGuide running locally on your device.
These steps make it sound more complicated than it is. It’s really just a matter of picking a viewer app for iOS (which may not be necessary on Android – I haven’t tried), and then transferring offline iGuides to your device for viewing.