string FilePath = HttpContext.Current.Server.MapPath("~/FILENAME.txt");
string FileContent = "";
FileContent += "Text Here" + Environment.NewLine;
File.WriteAllText(FilePath, FileContent);

byte[] fileBytes = System.IO.File.ReadAllBytes(FilePath);

string FilePath = HttpContext.Current.Server.MapPath("~/FILENAME.txt");

string FileContent = "";

FileContent += "Text Here" + Environment.NewLine;

File.WriteAllText(FilePath, FileContent);

var file = HttpContext.Current.Server.MapPath("~/FILENAME.txt");

HttpContext.Current.Response.AppendHeader("content-disposition", "attachment; filename=NewFile.txt");

HttpContext.Current.Response.ContentType = "application/octet-stream";

HttpContext.Current.Response.TransmitFile(file);

HttpContext.Current.Response.End();

$.ajax({
    url: "/api/WebService/spPOImportGenImportPOTxt/" + $("#ID").val(),
    type: "POST",
    contentType: "application/octet-stream; charset=utf-8",
    error: function(request, status, error) {

        alert("Something went wrong: " + error);

        $.unblockUI();

    },
    success: function(data) {

        alert("Data has been successfully saved.");

        console.log(data);

        $.unblockUI();

    }

});