PlayerPrefs Storage Location on Android or iOS

On Android they are stored in your phone's internal memory in /data/data/appname/shared_prefs/appname.xml Where appname is the full package identifier e.g. com.mycomany.example
No idea about iOS sorry

What I found out so far is:

"To be able to see the PlayerPrefs which Unity 3D stores on your Android, you'll have to have your device rooted. Rooting is necessary to be able to access the data - Partition. You can find the PlayerPrefs' XML file here: /data/data//shared_prefs/.xml"

Source

On iOS devices, the PlayerPrefs can be found in your app's plist file. It's located at:

/Apps/ your app's folder /Library/Preferences/ your app's .plist

I used a free MacOS app called iExplorer to find and verify any changes I make to the PlayerPrefs on my iPad app. If anyone has a handier/better way, please share.

I'm actually don't use PlayerPrefs. I prefer use what I've learned on this Live Training http://unity3d.com/learn/tutorials/modules/beginner/live-training-archive/persistence-data-saving-loading . It's been working for me so far.