iOS 8 Reset Can Erase Your iCloud Documents

According to Macrumors, using Reset all settings would result in all of your iCloud documents including Pages, Numbers, Preview documents and other kinds.

This happen when you go to Settings -> General -> Reset -> Reset all settings.
Even though the popover says "This will reset all settings. No data or media will be deleted", the phone system is going to erase your online documents. 

So to be safe, back up your documents using your Mac running OS X Yosemite. It is because only OS X Yosemite has the feature to access iCloud Drive directly. And that's how you can copy and paste all of your iCloud documents to a safe location.