Print it out on acid-free paper with a stable, acid-free ink. Have it bound as a hardback, and seal it into a waterproof container. Entrust it to one of your children, tell them to keep it in a safe-deposit box and take it out once a year to share with their children, and to pass it on to their children with the same instructions.
If you have it electronically, the absolute best case in 500 years is that it will be a relatively easy job for software archaeologists or historians to decode, assuming it's been periodically backed up to new media for all those years. The most likely case, though, is that in 150 years, the servers it was stored on, which have not been running for 80 years, will be picked over and/or melted down for precious metal contents by a tinker who wanders between mud-hut villages repairing their ancient metal pots in exchange for dried fish.
If you have it electronically, the absolute best case in 500 years is that it will be a relatively easy job for software archaeologists or historians to decode, assuming it's been periodically backed up to new media for all those years. The most likely case, though, is that in 150 years, the servers it was stored on, which have not been running for 80 years, will be picked over and/or melted down for precious metal contents by a tinker who wanders between mud-hut villages repairing their ancient metal pots in exchange for dried fish.