Make it as an attachment and give it a filename when returning the result:
protected ActionResult ViewPdf(string pageTitle, string viewName, object model)
{
// Render the view html to a string.
string htmlText = this.htmlViewRenderer.RenderViewToString(this, viewName, model);
// Let the html be rendered into a PDF document through iTextSharp.
byte[] buffer = standardPdfRenderer.Render(htmlText, pageTitle);
// Return the PDF as a binary stream to the client.
return File(buffer, "application/pdf", "myfile.pdf");
}
What makes the file appear as attachment and popup the Save As dialog is the following line:
return File(buffer, "application/pdf", "myfile.pdf");