There are a couple different ways to search for text within a PDF on your iPhone. The first way is to use the built-in search feature within the iBooks app. To do this, open the PDF in iBooks and tap on the magnifying glass icon in the upper right corner. Type in the word or phrase you're looking for and tap on Search. iBooks will then highlight all instances of that word or phrase within the document.
The second way to search for text within a PDF is to use Siri. Simply activate Siri by holding down the Home button and say Search for [word or phrase] in [name of PDF document]. Siri will then pull up that document and highlight any instances of the word or phrase you're looking for.
If you need to copy text from a PDF, you can do so by using either method above to find the text you want, tapping and holding on it, and then selecting Copy from the pop-up menu. You can then paste that text into another document or app.