Download books from other websites
iBooks supports the ePub file format, PDFs, MP3 audiobooks, AAC audiobooks, and audible.com audiobooks. iBooks Author books are supported in iBooks 2.0 or later on iPad, iBooks 4.3 or later on iPhone and iPod touch, and on all versions of iBooks for Mac. Audiobooks are supported in iBooks 4.3 or later on iPhone, iPad, and iPod touch.
If you want to add books, audiobooks, and PDFs from outside the iBooks Store or audible.com to iBooks, they need to be DRM-free. You can sync them using iTunes 10.5.3 or later. Some PDFs might not be compatible with iBooks.
Books that you download from the Kindle Store and other books that aren't in a DRM-free standard ePub format aren't compatible with iBooks.
Source:
https://support.apple.com/en-us/HT201478