To play the original levels, music, and graphics officially, you must own the game data files (IWADs). These are available for purchase on Steam and GOG.
If you already own the recent re-release, Bethesda provides free high-quality community content: DOOM II Free Download
While the game content costs money, the is open-source, allowing developers to create free "Source Ports" that modernize the game on new hardware. To play the original levels, music, and graphics
The most popular and versatile source port, supporting advanced graphics, modern controls, and heavy modding. To play the original levels
Historically, only the first game, DOOM (1993) , had a free nine-level shareware version. DOOM II was never released as shareware. 2. Free Official Add-ons & Updates