How much data are we talking about?

I'd agree with Alex that diversity is key, and I'd add, continually update media. I would not expect to be able to store stuff on a CD, put it in a draw, forget about it for decades, and still be able to read it when I needed to.

I keep most of my "live" data, along with archive data, on a solid state drive in a desktop PC. I have a script that runs nightly to compress and encrypt it and write it to a spinning disk drive in the same machine. It writes another copy to a micro-SD card. Yet another copy is written to DropBox, where it syncs to my phone. From time to time I swap the micro-SD in the desktop for another one I keep in my wallet. Sometimes I make another copy onto a USB drive.

So I have multiple copies, on multiple media, with varying eases of accessibility. DropBox counts as off-site backup in case my house gets hit by a meteorite or something. The other copies should survive if DropBox go out of business. Hopefully both those things won't happen at the same time.

If you are taking this route then you need to think about encryption and password management. I wouldn't trust a company like DropBox not to peek at the data I store there, nor would I keep it unencrypted in my wallet. The downside of encryption is the risk of forgetting the passwords, or losing access to the software that does the decryption.

Which is a general point: you need to be careful about file formats. Plain text is best for longevity. PDF seems pretty safe now, but might not be in 20 years. Archiving a copy of a PDF reader may help, but even that might not run on future computers. You may need to convert formats as new technology becomes available and old technology becomes obsolete and stops working.

For my needs, current technology is pretty good. A micro-SD card is tiny, and can store 64 Gb which is plenty for me. I also have good internet access.