We found one verified source offering a free activation key for Reaconverter 7 Standard:

Because it is a premium utility, the developers (reaSoft) require a paid license key to unlock the standard and pro features beyond the basic trial limitations. The Risks of "Free Verified" Activation Keys and Cracks

If you’re looking for a legitimate way to use ReaConverter:

If you do not want to purchase a Standard license, the developers offer reaConverter Lite , which is completely free for basic use. It works entirely offline for maximum privacy.