(1) create static documents in the simplest and most standard lossless (as far as possible) format.
(2) store them on a durable enough, yet simple enough medium. Today that would probably be something simple like an USB stick.
(3) transfer to new medium periodically, and take the opportunity to convert obsolete media files to new formats, if needed (you could keep the originals and each 'generation' as historical reference).
(4) train your children, grandchildren, etc to perform (3) and perhaps to rewrite the instruction in their own words (to cater for technology and language changes)
This also allows for multiple copies for easy backups and to entrust to each descendant individually.
