Some sites will ask you to "verify your humanity" by entering credit card details or phone numbers. A truly free collection will never ask for payment information.
While the original collection might be legitimate, the mirrors and sites hosting these files often aren't. Searching for "free verified" downloads often leads to several risks:
Often, these "free" links are just empty files or random data renamed to match your search query to drive traffic to ad-heavy websites. How to Stay Safe While Searching
If the collection is code-based or open-source.