Have you tried a "free activation key" that failed? Share your experience in the comments (and then go run a virus scan).
A legitimate Foxit PhantomPDF activation key is typically 24 characters long and can be applied in several ways: foxit phantompdf 101 activation key
Are you trying to recover a key you previously bought, or are you looking for a way to try the software for the first time? Where is the license information of Foxit PDF Editor? Have you tried a "free activation key" that failed