<WebBrowser Name="_pdfViewer" Source="D:/pdf/LargeFile.pdf"/>

<WebBrowser Name="_pdfViewer" 
            Behaviors:WebBrowserUtility.BindableSource="{Binding FilePath}"/>

public static class WebBrowserUtility
{
    public static readonly DependencyProperty BindableSourceProperty =
        DependencyProperty.RegisterAttached("BindableSource", typeof(String), typeof(WebBrowserUtility), new UIPropertyMetadata(null, BindableSourcePropertyChanged));

    public static String GetBindableSource(DependencyObject obj)
    {
        return (String)obj.GetValue(BindableSourceProperty);
    }

    public static void SetBindableSource(DependencyObject obj, String value)
    {
        obj.SetValue(BindableSourceProperty, value);
    }

    public static void BindableSourcePropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        var browser = o as WebBrowser;
        if (browser == null) return;
        var uri = e.NewValue as String;

        browser.Source = !String.IsNullOrEmpty(uri) ? new Uri(uri) : null;
    }
}