You will need to implement the search functionality yourself.
Searching for a text in a file or string is not too difficult.
Load the file with File.ReadString and then use String.IndexOf to find the location of the search query.
You can use Regex object for more powerful searching.
These are so well in the APK file I don't send them separately
Can I still use "load the file and then use with File.ReadString String.IndexOf" and/or Regex?
Do you want to allow searching for any string? Or only a determined set of terms?
There are several possible solutions.
I would start with a database solution. Convert your files to a database. Make sure that there is an index on the text column. You can have the file name stored as an additional field. This will allow you to find the files that contain the term.