I have scaned pdf file (88-page) 23MB in size (downloaded this way). Trying to reduce the size of the file I use some on-line web-page and it did a good job. PFD file went from 23MB to 13MB looking at both files there is not difference in quality.
I was trying to duplicate the result via linux command linux command line using "convert" "gs" etc, I can not even come close. -- Thelma