If you use iBooks to organize and hold your books and PDFs, there are lots of useful tools at your disposal to make finding what you're looking for even easier. One of those tools is the ability to search any book or PDF in its entirety for certain text.
Launch the iBooks app from the Home screen of your iPhone or iPad.
Open the book or PDF that you'd like to search for text within by tapping on it.
Tap on the magnifying glass in the upper right.
Type in the term you'd like to search for and then tap Search on the keyboard.
You'll now see search results you can scroll through for that word or term. Just tap on the one you'd like to jump to.
That's it. You can use the search feature to search either single words or whole strings of words.