Copper aint ideal but does meet specification to carry compressed air. In most instances where we have fitted compressed air lines we use galvanised steel tube with screwed fittings, but that's in a commercial premises with more chance of mechanical damage....

If you are going to use copper, might be an idea to copper/copper braze or silver solder the joints rather than soft solder, as said above the soft soldered joints can suffer premature failure....

HTH

Ian