They don’t require a subscription or association with an institution (accessible) and allow you to do an advanced full text search, IIRC (good).
A good thing about FileLocator is that you can tell it to display the lines around the search results and with that you can quickly skim through the results and see if they are relevant, without having to open each document.
An ideal search engine I think would be one that allowed you to say: search for any instances of “ground station,” “regulation,” “electrical,” within #### characters of each other. I’d assume any hits would talk about electrical regulation of ground stations then for example.
Edit: this tool allows the boolean NEAR operator so you can do this. In the configuration menu you can also change the “NEAR distance.”