How to make Finder search within the contents of files
I'll put some pictures to the reply answers given.


Add something to the search field to get the search options to show up. I just used a space in this example.
Click the + button to the right of save.

Choose contents from the left drop down menu and then type in your search term. You may need to clear out whatever you added in the main search field to not limit your results.

You can instead type content:search term in the main search field.