Valve has occasionally made the full game free for limited periods, such as during the "Learn with Portals" initiative. While not currently active, these events are worth watching for.
While Portal (2007) is a legendary title by Valve , finding a for the full PC game today requires navigating between official demos, historical giveaways, and third-party sites. Ways to Play Portal for Free (or Cheap) portal-1-pc-game-free-download
If you already own the game, the high-end graphical overhaul Portal with RTX is a free download that adds modern ray-traced lighting and high-res textures. Valve has occasionally made the full game free