We are from Us and every summer perform in CANNES ... we are ready to buy a PA 800 keyboard .... It will work the AC adaptor in US? or do I need to buy a converter?

Check if the adapter is "universal" or not (or it's told 110-240 V or near these, and 50-60 Hz in the specs), if it is, it'll work, you'll eventually need only either an adapter to change the plug to the mains itself of find the relevant local cable (in computer shops/supermarkets usually).

If it's not, well, you'll have to use a power adapter in between. Just check it's rightly stepping down or upping (or both) the Tension AND if possible the frequency though digital stuff is not prone to issues with this. Another solution is to buy a small US power conditionner second hand that'll handle the conversion of both, it should be universal (check), and you'll get proper 110V on the other side.