AnytxtSearcher is a local full-text search engine that finds content inside your files in seconds.
It can search the contents of many document formats — Word, Excel, PowerPoint, PDF, and more — with built-in preview. Even Markdown files can be searched, though that option is off by default. To turn it on, go to Settings → Index Management → Index Rules, click "Add", type the .md extension, confirm and save, then rebuild the index. After that you can search inside your Markdown files just like any other document.