您不能强制文件下载到的地方。

You can't force the place the file is downloaded to.

在您的code，你不应该写入文件，但对 的OutputStream 从响应。

In your code, you shouldn't write to file, but to the OutputStream from the response.

RenderingResult result = reportProcessor.RenderReport("PDF", instanceReportSource, null);

HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=\"file.pdf\"");
HttpContext.Current.Response.OutputStream.Write(result.DocumentBytes, 0, result.DocumentBytes.Length);