As the Codex is a community-managed Wiki I don’t think there is a downloadable version but I will ask some other folks just in case.
Unfortunately i believe the WordPress Codex will be unavailable to download simply due to the nature of how that information is stored.
Like WordPress, the data is stored and retrieved via a MySQL database, short of the WordPress team sharing database contents with it’s users, it’s highly unlikely the information will be available publicly beyond the pages you can access now.
I cannot speak on behalf of all the team involved, but i can tell you that it’s not likely to be available due to the nature of how that information is stored.
If you’re curious, the codex is run using MediaWiki.
