Convert a pdf and decrease size

First:
convert  -density 200 -trim AS1.pdf AS2.pdf AS3.pdf AS4.pdf AS_all.pdf
And then "Print to file". That's it. It decreased a 25MB file to 6.7MB. Pretty good.
P.S. That's an interesting one, but obviously it works only with 1 page:

convert -density 600 test.pdf -background white -flatten -resize 25% test.png