One of the issues why I prefer to create lot may smaller docs is becasue the finder finds notes based on titles and not content of the notes. (I hope I am correct, the Mac OS search is not deep enough and is restricted to titles)
Is there any tool which can help me search notes based on its content?
Something I often forget is that searching with Spotlight (the simple UI that is by default opened with Cmd-Space) will return some, but not all, of the results you’ll get from searching Finder. If you can’t find what you’re looking for, try Spotlight results > Related Searches > Search in Finder.