Can you boil juice down to powder?!

If you boiled juice you would have a syrup because the water would start to evaporate away leaving the sugar and other compounds behind. If you continued boiling for long enough you would eventually burn the syrup and end up with a black charred mess. Salt doesn't burn but sugars and other flavor compounds do. I guess if you did it just right you could continue heating the syrup and then at the right time let it cool off. It would harden and then you could just break or grind the solid up into a powdery substance.

No, to boil is to make something evaporate and evaporation is when something, namely a liquid, becomes a gas or sometimes becomes a syrup.