How to prevent users from downloading videos?
Unfortunately, no, that's the entire reason DRM exists.  It is a bit like trying to prevent someone from recording an on-air broadcast.  When you send video data over the internet to someone's player, they can simply store the information being sent to the player unless you obfuscate it and make it so that the player will only work under certain circumstances and will not share the data.  This, by definition, is DRM.

What DRM attempts to do is control the reading of the data entirely, so that it can not be copied.  This has varying degrees of success and rarely, if ever, works particularly well.  It may keep honest people honest, but if you are sending someone data in a way that they can access it, measures to try to stop them from copying it are... difficult.  The most advanced systems use special display drivers and encrypt the data right up to the point it is being displayed on the screen (HDCP), that way other software on the computer can't directly pull the information off the frame buffer being prepared for the screen.

There may be some ways you can mildly obfuscate the access to your video, but ultimately, if you send it in the clear, it is trivial for a knowledgeable viewer to store the datastream.  If you use DRM, it is substantially harder, but still likely to be able to be worked around by a dedicated attacker.