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//First method
var filepath = Server.MapPath("~/Files/Filename.pdf");
using (MemoryStream stream = new MemoryStream(pdf))
{
FileStream fs = new FileStream(filepath, FileMode.Create,FileAccess.Write);
stream.WriteTo(fs);
}
//Second method
var filepath = Server.MapPath("~/Files/Filename.pdf");
FileStream fs = new FileStream(filepath, FileMode.Create,FileAccess.Write);
fs.WriteByte(pdf);