For anyone interested,
thanks to @tbrown313, I installed OCRmyPDF with homebrew (HERE is the instruction).
to automatically scan the incoming PDFs I created the following hazel rule:
the embedded script is quite simple:
ocrmypdf -l deu+eng "$1" "$1" --skip-text
the --skip-text jumps over every page which already contains text (scans from my iPhone already contain a OCR layer).
you can change the languages (in my case German and English) according to your personal needs.