Downloadable content only for subscribers?

All of this can be done with code of course but since you say,

“The first part (check if the user is registered / subscribed) is the difficult one for me.”

…then your time will be better spent going directly to plugins that allow for similar functionality as described in your question.

The first recommendation,

WishList Member [premium only]

This is a full-fledged membership plugin, while its not the only one of its kind, its good.

Easy Digital Downloads [free with optional premium add-ons]

This plugin, while smaller out of the box compared to the above, is focused more so with, downloads as its name suggests. It’s developed by fellow WPSE user Pippin.

I’m not overly big on recommending plugins, but when I do I try to recommend those of which I have used and think are good quality (unless stated otherwise).

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