Make a file open in browser instead of downloading it

Thanks to all the answers, the solution was a combination of all of them.

First, because I was using a byte[] the controller action needed to be FileContentResult not just FileResult. Found this thanks to: What's the difference between the four File Results in ASP.NET MVC

Second, the mime type needed to NOT be a octet-stream. Supposedly, using the stream causes the browser to just download the file. I had to change the type application/pdf. I will need to explore a more robust solution to handle other file/mime types though.

Third, I had to add a header that changed the content-disposition to inline. Using this post I figured out I had to modify my code to prevent duplicate headers, since the content-disposition was already being set to attachment.

The successful code:

public FileContentResult GetDocument(string zipCode, string loanNumber, string classification, string fileName)
{
    byte[] doc = _docService.GetDocument(zipCode, loanNumber, classification, fileName);
    string mimeType = "application/pdf"
    Response.AppendHeader("Content-Disposition", "inline; filename=" + fileName);
    return File(doc, mimeType);
} 

It looks like someone else asked a similar question a while ago:

how to force pdf files to open in a browser

With an answer saying that you should use the header:

Content-Disposition: inline; filename.pdf