<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition />
    </Grid.RowDefinitions>
    <telerikPdfViewer:RadPdfViewerToolbar PdfViewer="{Binding Source={x:Reference pdfViewer}}">
        <telerikPdfViewer:ZoomInToolbarItem />
        <telerikPdfViewer:ZoomOutToolbarItem />
        <telerikPdfViewer:NavigateToNextPageToolbarItem/>
        <telerikPdfViewer:NavigateToPreviousPageToolbarItem/>
        <telerikPdfViewer:NavigateToPageToolbarItem/>
        <telerikPdfViewer:FitToWidthToolbarItem/>
        <telerikPdfViewer:ToggleLayoutModeToolbarItem/>
        <telerikPdfViewer:PdfViewerToolbarItemBase Text="Save" Command="{Binding SavePdfDocumentCommand}" />
    </telerikPdfViewer:RadPdfViewerToolbar>
    <telerikPdfViewer:RadPdfViewer x:Name="pdfViewer" Grid.Row="1"/>
</Grid>

public partial class MainPage : ContentPage
{
    public ICommand SavePdfDocumentCommand { get; set; }
    public MainPage()
    {
        InitializeComponent();

        this.BindingContext = this;

        Func<CancellationToken, Task<Stream>> streamFunc = ct => Task.Run(() =>
        {
            Assembly assembly = typeof(MainPage).Assembly;
            string fileName = assembly.GetManifestResourceNames().FirstOrDefault(n => n.Contains("pdfviewer-overview.pdf"));
            Stream stream = assembly.GetManifestResourceStream(fileName);
            return stream;
        });
        this.pdfViewer.Source = streamFunc;

        this.SavePdfDocumentCommand = new Command(this.SavePdfDocumentCommandExecute);
    }

    protected void SavePdfDocumentCommandExecute()
    {
      //custom implementation for saving the document
    }
}