Page Not Found errors are also known as a a 404 error page. It happens when a visitor to your site tries to access a page that doesn't exist on your site for one reason or another. These errors typically occur when someone mistypes a URL on your site, when there’s a broken link from another page, when a page that previously existed is moved or removed, or there is an error when a search engine indexes your site. For a typical site, these errors account for c. 3% of the total page views, but they’re often untracked by traditional analytics platforms like Google Analytics.
Page Not Found/404 errors would be something you would want to look at working with your hosting provider to correct. If you would rather have Cloudflare help you monetize your website's 404 errors, please look at our SmartErrors app.