By default Desktop8.ndk file is originated in C:\Program Files\IBM\Notes\Data.
Desktop8.ndk is a kind of database which kept the cached copy of those items in databases that were accessed by the user in Bookmark. Cached items which are found in this file are mainly the design elements along with their UNIDs and NotesID.
In the prior editions of Lotus Notes, desktop8.ndk file was characterized as desktop.dsk. As IBM Notes new versions came out, the file name was also updated as desktop5.dsk, desktop6.dsk and after the release of IBM Notes 8.5 this file is named as desktop8.ndk