Is it possible to have the iOS and OSX documentation available for offline reading? If so, how?
A lot, if not most, of the online documentation has a link to a PDF of what you're reading in the top right of the window, underneath the Search icon.
The Documentation for iOS is no longer a part of SDK for offline reading - looks like you need to install from preferences of XCode, select Documentation preferences - click get next to 'iOS 4.3 Library Documentation' to install it.
When in XCode press ⌘ + ⌥ + ⇧ + / then start searching for methods or classes you're interested in.
As of Xcode 5, you can choose to download sets of documentation under Preferences\Downloads Tab\Documentation.
As of Xcode 7+, you can find the download for the documentation in Preferences > Components.
You have to download documentation for every OS (macOS, watchOS, iOS, tvOS) separately.
Go to Xcode's documentation preferences, and make sure that any documentation set you're going to need offline doesn't have a Get button next to it. If it does, click the button to download the set while you're online.