We should never use free tools for plagiarism analysis. It is a serious issue because people don't even focus on this aspect. Many free plagiarism analysis tools have a backup system. They will backup your article and after few days or even few hours, The article is published on the website. Yes! Your article will be published on another website owned by them. That's why you should use those sites with caution. In some cases, the article get's indexed on their site before yours. In this way, your site will become the copy content one. This is a serious issue and that's why I prefer paid tools for this purpose.