Due to copyright, I cannot supply the PDF directly. However, you can often find The Nature of Explanation (1943, Cambridge University Press) via:
This is the most reliable free source. Search for "The Nature of Explanation Craik" on the Internet Archive. They often have scanned copies from university libraries available for borrowing or download in PDF, EPUB, and DJVU formats. You may need a free account to "borrow" the digital copy for one hour or more. kenneth craik the nature of explanation pdf