Hello,
The memory limit is not the problem in your case ; but the "max execution time".
It means that if the user haven't download the file within 60 seconds, the server will shut down the PHP process and the user download in the same time.
In order to secure the download, the server need PHP or a specific module.
So if you want to deliver large files and PHP do not give you the possibility ; you'll need to use server systems like "X-Transfer" or "mod secure download".
These are features/modules for Web Servers, and if your hosting do not provide them, I'm afraid that it will be complicated for you to provide secured downloadable files.
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.