I am first getting the file bytes
byte[] fileBytes = File.ReadAllBytes(destinationFilePath);
And then push the file out
return File(fileBytes, FileContentType.ApplicationOctetStream, fileName);
For small files this is great but for big files like 20mb and above, the conversion takes time.

How can i make it better so that conversion time gets better?

Is there some third party component available that can do the job better?