Downloading ROMs for games you do not own is a legal gray area. Always ensure you are following local copyright laws and supporting developers by purchasing official releases when available.
Unlike later "Free-to-Play" iterations on mobile, the DS version is a complete, standalone package with no ads or "energy" timers. cut the rope ds rom
Using software like DeSmuME (PC) or Drastic (Android) allows you to play the game with enhanced resolution. Downloading ROMs for games you do not own