Any test I've done with Zen Cart and MP3/PDF files on iOS devices has downloaded the files flawlessly. Of course, you have to let them download as .mp3 or .pdf and not as .zip
There are some iOS apps capable of opening .zip files, but that won't accomplish the end result you're seeking, as it would require them to open the downloaded zip specifically into another app, and many users won't "get" that concept until they've been shown how to do it. So, that's NOT the way to go.
The right way is to serve your downloads in native file format, not zipped.
As for your files being opened in native apps on PCs, perhaps you need to change your /pub/.htaccess file and remove the # from these two lines as highlighted in red:
Code:
# tell all downloads to automatically be treated as "save as" instead of launching in an application directly
# (just uncomment the next 2 lines by removing the '#'):
# ForceType application/octet-stream
# Header set Content-Disposition attachment
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