Others have already mentioned other cloud storage options, which are good because they both synchronize your writing and also give you a backup in case something goes wrong.
If you already have a backup solution, you could consider syncthing. It keeps folders on several different devices up to date without storing a copy in the cloud somewhere. It's free but takes some setting up
You could also compress your files into a zip, add a password, and then store it in google drive. You can't do online writing and it'd be a hassle to re-encrypt them each time you make a change but google wouldn't be able to scan your docs and you could restore your writing if something goes wrong on your local computer
While waiting for someone who's more familiar with Voltaire, Nicholas Cronk's interview provides some context to Voltaire's career and gives some book recommendations. The two primary works he recommends are Candide and A Pocket Philosophical Dictionary. He explains his reasoning for both pretty well.
Fwiw, if you're just looking to start reading Voltaire, I don't think you'd go wrong reading the Candide. It's an enjoyable, short, and funny satire. The only context you may want first is Voltaire is largely responding to a view around in his time commonly called 'the best of all possible worlds'. This article explains that a bit better
If you're looking specifically for his philosophy, the SEP contextualizes what Voltaire has written and where. Its bibliography has primary sources, so you could always start with the one that you find most interesting