Hi,
It could come from several things.
If the files are big, it could come from a limitation of your hosting, like a memory limit or a max execution time limit.
So try with a small file to confirm that.
If it doesn't work with a small file, then the problem is likely to be an encoding or gzip compression issue.
Try turning off the gzip compression in the Joomla configuration, and make sure that you don't have some specific rules in your htaccess to force the compression based on the file extension, or similar rules in your apache server settings.
It could also be a warning/notice from a third party extension so turn off the "error reporting" in the Joomla configuration.
One thing is sure is that it's not HikaShop itself modifying the file when sending it to you.