If your website is accessible to the public, it has probably already been indexed by search engines, such as Google. At the time of this writing, Google is the most popular search engine by a very large margin. Google offers a free service that allows your visitors to search your site via Google. This is the simplest option. Other options include my own simple website search engine, among other pieces of software you can install on your own site to provide a search that does not mention or promote another company. However, these tools do require that you have some basic system administration skills, and the willingness to run an indexing program regularly to keep your search engine's database up to date. If that does not sound realistic for you, I recommend that you embed the free Google search or a similar service.
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