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.