I like to make converters like https://www.pdf.to and random one off ideas to see if any hit like https://www.microphonetest.com
I don't do any SEO/Ads, although one of the noteable links came from paying a outreach company to reach out to reporters and one wrote an article https://www.lifehacker.jp/article/lht_pdf-to/, although that was many iterations ago. Just been focusing on improving the functions and design since that initial launch phase back in 2019
edit: Oh! You're the guy that runs Yout