Hi,
As I said before, you can't use the customer download area for that.
Even with the custom order field, you won't get the file you upload in the customer download area. You'll get it in the order details when you open the order via the order listing of the frontend.
Note that as per your server restrictions, you can't upload files bigger than 20M. Maybe it comes from that that it doesn't work ? Did you try with a small file ? Also, you're limited to the extensions that you allowed in the Files section of the HikaShop configuration so please check that.