Below is a sample code that lets you to create a text file within the application local folder in Windows Phone 8.1.
How to Create a Text File within the Application in Windows Phone 8.1 ?
var localAppFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
var fileHandle = await localAppFolder.CreateFileAsync("AbundantcodeData.txt", Windows.Storage.CreationCollisionOption.ReplaceExisting);
await Windows.Storage.FileIO.WriteTextAsync(fileHandle, "Abundantcode is one of the popular programming website");