Reduce file size of pdf text file scanned with app

I had scanned 11 sites of a document in black and white and exports it as an pdf file. With two different known apps I got a file size of about 8MB. I tried different solutions with ColorSync filter or ghostScript commands but the pdf size does not shrink noticeable.

With scans from a printer I got pdf files with about 10 percent of that size without loss of quality. Does anyone knows how to reduce a large pdf file?