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It's an orange app with a white open book icon. Select the e-book or PDF you'd like to share. It's in the upper-left corner of the screen. On some PDF files, this button will not appear. It's the square icon with an upward-pointing arrow. Depending on the publication type, it will be in either the upper-right or upper-left corner of the screen. You may need to scroll left to see all the options, such as email, text, AirDrop, or social media. Tap a button to select a method.  Recipients will receive a link to purchased e-books in the iTunes Store. Recipients will receive entire PDF files. Email is the most efficient way to share a PDF file.
Open the iBooks app. Tap a book. Tap ⋮≡. Tap the Share button. Choose how to share the book. Share the book.