How to get Apples Human Interface Guidelines as pdf (latest version 2015)?
iBooks has changed the download location to the "Desktop" for I don't know which reason. I found the iOS Human interface.epub file and loaded it with the iBooks Author app.

There I went "File" - "New from ePub file..." . After the app loaded the file I went for "File" and "Export" and finally as PDF.

I also tried downloading the .epub file onto iBooks on my iPad. There is no export option available and the mailing open only mails the link from the iTunes store and NOT the .epub file itself. Hopefully this helps someone... 

Found the solution by accident , since I didn't expect the .epub file to be on my desktop...