public ActionResult SomeAction()
{
    //Call your existing partial view and convert it to string
    var html = RenderToString(PartialView("SomeView"));

    var inputStream = new MemoryStream(Encoding.UTF8.GetBytes(html));
    var outputStream = new MemoryStream();

    var documentConverter = new DocumentConverter(inputStream, DocumentFormat.Html);
    documentConverter.ConvertTo(outputStream, DocumentFormat.Pdf);

    //Rewind the stream which contains the converted Pdf now.
    outputStream.Position = 0;

    return File(
        outputStream,
        System.Net.Mime.MediaTypeNames.Application.Pdf,
        "Result.pdf");
}

private string RenderToString(PartialViewResult partialView)
{
    var rc = Request.RequestContext;
    var controllerName = rc.RouteData.Values["controller"].ToString();

    var controller = (ControllerBase)ControllerBuilder.Current
        .GetControllerFactory().CreateController(rc, controllerName);

    var controllerContext = new ControllerContext(rc, controller);

    var view = ViewEngines.Engines.FindPartialView(controllerContext, partialView.ViewName).View;

    var sb = new StringBuilder();

    using (var sw = new StringWriter(sb))
    using (var tw = new HtmlTextWriter(sw))
    {
        view.Render(
            new ViewContext(controllerContext, view, partialView.ViewData, partialView.TempData, tw),
            tw
        );
    }

    return sb.ToString();
}