iBooks is a great way to download books and PDFs straight to your iPhone and iPad as well as manage them. If you frequently read or download titles that you don't plan on reading again, you can always delete them to remove some clutter from your virtual bookshelves.
Launch the iBooks app from the Home screen of your iPhone or iPad.
Go into the collection or section you'd like to delete PDFs or books from.
Tap on the Edit button in the upper left hand corner.
Tap on the titles of the books or PDFs you'd like to delete.
Now tap the Delete button at the top towards the right.
You'll be asked to confirm once more that you'd like to delete the selections from your iBooks library. Tap on Delete one more time to confirm.
That's it. The selected titles are now removed from your iBooks library. If you ever want to re-download a book that you've purchased, you can do so by visiting the iBookstore again and reinstalling it.