How to locate a file in Spotlight using folder and file name?
If you're comfortable opening the terminal you could run the command

locate ABC/readme.txt


and it will give you the full file path to any files on your machine that are named readme.txt in a folder named ABC.

While you're there you can copy and past the path as an argument to the open command and it will open it for you.

Taking it one step further you can use the output of locate as the arguments for open with xargs

locate ABC/readme.txt | xargs open


Will open all files that have ABC/readme.txt on your machine just as if you had double-clicked them.