public static class MyHelpMenu {

    [MenuItem("Help/My User Guide")]
    public static void OnHelp_MyUserGuide() {
        EditorUtility.OpenWithDefaultApp("Assets/My Asset/Support/User Guide.pdf");
    }

    [MenuItem("Help/My API Documentation")]
    public static void OnHelp_MyAPIDocumentation() {
        // Note: Open CHM file on Windows, otherwise use online guide
        if (Application.platform == RuntimePlatform.WindowsEditor)
            EditorUtility.OpenWithDefaultApp("Assets/My Asset/Support/API Documentation.chm");
        else
            EditorUtility.OpenWithDefaultApp("http://example.com/product/api");
    }

}

I would assume it's as easy as

[MenuItem ("Help/Show PDF")]
static void ShowPDF()
{
    System.Diagnostics.Process.Start("yourpdf.pdf");
}