Users can upload ePub and PDF files (DRM-free only) for slicing and remixing.
ePub files: To avoid slicing errors we strongly recommend you validate ePub files before slicing. Validating is easy, and free. Go here to upload your file and validate.
PDF files: PDF files must be bookmarked to slice. The bookmarks are used to indicate where you want slicing to happen. For example, if you want to slice a book by chapters, you add a bookmark at the start of each chapter. Go here for more information about bookmarks in PDFs.