While the lure of a "free download" is strong, is a game built on data and longevity. To ensure your monsters grow strong and your save files stay safe, sticking to the official Steam release is the way to go. Pro-Tip for New Trainers:

Monster Rancher 1 & 2 DX PC: Is a "Free Download" Worth the Risk?

If a friend owns the game, they can share their library with you for free!