I think it was Okular. I was looking for a way of editing PDFs without ads, invasive software, or subscription based payments.
Free and Open Source Software
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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OpenTTD. Sure, I had used android, but it was the first thing I consciously knew was open source and enjoyed.
OpenFOAM. I needed powerful software to do CFD that was free, as in free beer, and found OpenFOAM. To run it on windows, the installation instructions first step was install linux on a vm then follow the linux installation instructions. I did that and started using the vm for other stuff until I found myself using the vm for most tasks, but kept using windows for gaming. To learn more, I got myself a pinebook, which replaced most of the usage of my vm. When windows decided to self destruct, I had learnt enough to install and configure arch with minimal help. Now, nothing can persuade me to go back.
For me, the gateway was via palm pilot careware. My dad had a PDA when I was a kid, and he let me learn how to program it. Then I learned that there were websites to download software for it, and some of that software was "careware", ie pay only if you're able. Something clicked in my head that I could both write and access software without cost being a barrier, and that got me reading about FLOSS philosophy as I entered high school and suddenly I was dual booting ubuntu on the intel iBook I had saved up for and then it was too late for me: FLOSS had me.