Hi,
Indeed, for security reasons (so that no one can download the file with just the link to it), the files uploaded in HikaShop are not directly accessible from the outside. So, the only way to access them is to go through HikaShop's code which checks that the user has the right to download it, then load that file in memory and send its content directly to the browser.
So, if you want to allow the download of files in HikaShop, your PHP memory limit needs to be above the size of the files (+ the memory used by joomla and hikashop ).