Amazon's KDP platform for self-publishing actually allows authors to upload a MS Word file. If you have a novel that is all text, and you use Word's formatting styles consistently, KDP does a good job of converting the files. If you want new chapters to start on a new screen, for example, you need to be sure the word style for the chapter name or number creates a new page in print. But of course, you can't really tweak the format easily. If you want to change something, you need to go back to Word, edit the file, reload it to KDP, and look at it again. If you have the kind of book that has fancy formatting or lots of images, this can be tedious.
As first reported by The Digital Reader, Amazon has launched a beta version of a new tool called Kindle Create. The Talking New Media blog checked it out, too, and reported on their first impressions.
The new tool is different because you download it and run it locally, o your PC or Mac, and then preview the results from within the app itself. It takes over 500 MB of disc space, but hard drives are pretty big these days.
Although the Amazon page only mentions Word files, the program will accept PDFs, but there was some discussion in the comments on the Talking New Media post that suggetsed it wasn't really converting the PDF but just wrapping it in code that made it into a "print replica" not a true ebook.