Official user manuals for the iPhone 5s are available in PDF form from Apple's Support website. The user guide needed depends on the operating system installed on the phone.
You can also read the user guide on your mobile device for free through iBooks, included with iOS 8 and above.
The iPhone 5S was originally supplied with Apple's operating system, iOS 7, which was released on September 20, 2013. The operating system is upgradeable for use iOS 8.4, released on June 30, 2015, or iOS 9.0, released September 16, 2015. User guides for each of these operating systems are found on Apple's Support website.