Hello,
Once you have the "attach as download" setting, you will get some text;
That text ( #hikaserial:pdfserial:1 ... ) should be use in the product ; you need to create a product file but instead of uploading a file or providing an URL, you put the special content.
And it will create a dynamic downloadable file for the product.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.