How can I search for text content in all my files?

I have a lot of text in diagrams.

I use the Mac desktop application and save the files in Dropbox.

Ideally, I would like to be able to search for this content by performing a Spotlight search or otherwise by another method.

Right now, everything I create in seems to be stored and forgotten, because unless I go to a file and look at each tab, I forget what I put there.

Any solution to this problem would be appreciated. Thank you.