A few days ago my sister emails me with a question about how to send files securely. I use a self-hosted version of NextCloud (think a private version of Dropbox on steroids). This would not work for my sister: 1) it costs money and 2) it would be to complicated for her to use.
I used to recommend Firefox Send. It was simple, easy and free. Unfortunately, Firefox Send was discontinued by the Mozilla Foundation in September 2020.
A quick search on the Interwebs turned up Wormhole. Their tag line is, "simple, private file sharing". Wormhole is a functional replacement for Firefox Send and the wormhole animation when you send a file is kind of fun.
I also recommend reading their brief article about why they built Wormhole (https://wormhole.app/about). It explains why services like Dropbox are not secure and why end-to-end encryption is important.