AC/DC "High" Voltage
It really doesn't matter whether you send AC or DC over the cable. If you need to reduce voltage drop, you'll have to use higher voltage or thicker cable. For a 300m cable it really makes sense to send the Mains voltage (220V) through it and install a PSU on the other end to produce 12V or whatever voltage you need. If you don't have Mains available, it may even make sense to produce it using an inverter.

PS: having high voltage drop in the cable is not only wasteful, but also dangerous for the load. Unless your load consumes quasi constant current (like an incandescent bulb), you can't realistically guarantee a stable input voltage for it. Even if you use two bulbs in parallel, your power line will likely fry one bulb should the second one burn out.