I should print this file on paper, but it is heavy (2,3 MB) and the printing process is very slow.

But this is not the whole problem. I should first create another .pdf file (grayscale) with four pages on each sheet. When I try to print this file as a .pdf file, the process of creation of the new file is even slower than the printing process, and the output file is much heavier than the original (hundreds of megabytes).

How can I shrink the pdf file? Is there a way to create a printer friendly .pdf version of the file? Before you ask, I can't simply print it on pages. I must create this .pdf file with four pages on each sheet.