How to Save a Mail Attachment to iBooks in iOS
You can save many email attachment file types directly from Mail app to iBooks in iOS, this allows for easy offline viewing on an iPhone or iPad, and it offers the benefits of using iBooks for reading and reviewing documents as well.
By saving an email attachment to iBooks, you actually convert and create a PDF file of the attachment in question, and iOS handles it all. For example, if you save a .doc file to iBooks, it will convert that to a PDF. Similarly, if you save a group of multiple images from an email attachment to iBooks, the collection of images will save as a single PDF file within iBooks.