: You can buy the verified, complete version of the game digitally on established PC storefronts like Steam or GOG .
: If you want to play for free legally, you can download the original 9-level shareware episode from the Doom Wiki Shared Files or play it directly in your web browser via emulation sites like DOS.Zone . Ultimate Doom Free Download
If you tell me what operating system you are using, I can guide you on how to get the classic shareware version running safely. : You can buy the verified, complete version