This did it for me, works like a charm:
yum install tesseract
Then pick you choice out of the many languages to be recognized and install them (English is installed by default): “yum search all tesseract”
and install the requered language(s) e.g. yum install tesseract-langpack-fra
Install the full text search OCR app in nextcloud, go to settings -> search and set your installed languages (watch the language abbreviation e.g. eng,fra,deu) and enable OCR.
Just not sure the interval when elasticsearch re-indexes, maybe this is related to the cronjob?
Here is a test image to upload and test https://courses.cs.vt.edu/csonline/AI/Lessons/VisualProcessing/OCRscans_files/bowers.jpg