Hello,
HikaShop "download files" are useful when you want to propose a file for a certain period of time and/or for a specific number of downloads after a product is bought.
HikaSubscription "plan files" are useful when you want to propose a file which can be download only when the customer subscription is valid ; like for example what we're offering with our components (when your HikaShop license expired, you cannot download the updates anymore ; you need to renew your subscription for that).
Afterwards, file downloads are downloads. There is no kind of link with Joomla article.
But you can use HikaSubscription to change the user groups so, to give them access to sections of your website depending the ACL you have configured.
That's something totally different and unrelated to downloads ; but it's a feature that HikaSubscription is providing.
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.