Is it necessary to be a Pro subscriber to Iconfinder to download icons?
The answer is no. You can of course purchase individual icons or icon sets by making a prepayment to your account balance (yes, you need to be registered as a user on Iconfinder in order to purchase icons).
How to make a prepayment to your account balance
In order to do this, please follow the steps described in this article: How to make a prepayment to your account.