In addition to Windows built-in Search indexing, I find free DocFetcher a very powerful indexed search tool. As you set it up, you define the folders (and all subfolders) to index and to be watched by docfetcher-daemon-windows.exe, which indexes new files dropped into those folders.
DocFetcher has a simple query syntax, can index a huge variety of file formats, in addition to plain text, and indexes are rapidly created.
The only disadvantage is that it requires Java to be installed. However, relying on Java enables this application to run on many platforms, including Linux, as well as on Windows. For example, in the image below, I selected to index everything in the Windows Documents folder, as well as other locations.