The trick is to convert the document into a string in base64 format first, and then use FileStream or File.WriteAllBytes() to add files. There are many ways on how to convert a document into a string in base64 format on the Internet, so there are not many bb here~

string fileBase64String; //This is the base64 format string of the document
string path = "c:\temp\document path\document name";
if(string.IsNullOrEmpty(fileBase64String))
{
  using (FileStream fs = System.IO.File.Create(path))
  {
   //using (FileStream is to avoid file lock
   //No need to write any code here

  }
}
else
{
  Byte[] bytes = Convert.FromBase64String(fileBase64String);
  System.IO.File.WriteAllBytes(path, bytes);
}