All iOS devices running iOS 3.1.2 and higher are susceptible to the exploit. The vulnerability is easily exploitable. The latest one-click, no-computer-required Jailbreak solution for iOS 4 devices uses this same method to break Apple’s own security. The easiest way to avoid this problem is by not going to any PDF links directly and not loading any PDF from any non-trusted source. You can also jailbreak your iPhone and install a program that will ask for authorization every time your browser encounters a PDF.”]
Source: https://gizmodo.com/apple-security-breach-gives-complete-access-to-your-iph-5603319