I downloaded a mobi file from a public forum but when I copied the file to my Kindle, I don't see in the list of books available for reading. So when I looked back in the PC, the files, I see a key marked in Green only in this file. I renamed the original file from the downloaded folder and copied to Kindle files folder. New copied and first copied, both files look have the green key in the mobi file icon.
I suspect this is some amazon file secure method to ensure that purchased books cannot be shared among others. But in the forum that I downloaded the file, many has commented that they are reading the book successfully.