ViewModel.cs

public ViewModel()
{
    Init();
}

private async void Init()
{
    var file = await Package.Current.InstalledLocation.GetFileAsync(@"Assets\Text.txt");
    Text = await FileIO.ReadTextAsync(file);
}

public string Text
{
    get { return _text; }
    set
    {
        if (value == _text) return;
        _text = value;
        OnPropertyChanged();
    }
}