You may be familiar with the Xcode Open Quickly option, which provides a shortcut to open a file in your project via Shift-Command-O and entering a file name. This is often much quicker than searching through the Project Navigator to find a file.
How to View System Header Files
Good news is, you can also use the same process to view header system files. As an example, bring up the Quick Open dialog and enter stdio – as you type a series of matches will be shown: