If you wish to make a handout out of a series of PDF slides you might want to make a “n up” version that is a version which has a number of the slides per page. The tool of choice is
pndfnup which is easily available for Debian and Ubuntu distributions.
pdfnup --nup 2x2 cfd_slides4.pdf
worked very nicely for me! Conveniently
pdfnup doesn’t overwrite the original file but produces a new one called: