Ubuntu – How to convert a scanned PDF into a PDF with text

I have scanned about 80 pages into gray scale pdf (image format).
The end size of the file is about 70MB, which is very huge.

Now I am looking for a method to convert the grayscale image-based PDF file into a simple black/white text-based PDF file.

I have done many attempts with gs but with no success (only a few percent recovery).
If any expert has some idea, kindly let me know.