Shooters!
How to Download The Doom Demo Right Now
Steam
These are the steps for PC gamers who use Steam:
Open Steam
Search "DOOM Demo"
Select the second option (unless Masquerade: The Baubles of Doom Demo is not your first option)
Click "Download Demo" in the sidebar.
Alternatively, to get it through a PC browser, go to this link and click "Download Demo" in the side bar.