I don't know about Windows, but under Linux you can do this:
From the command line write:
pdfimages pdffilename.pdf f
You will get the pages from the pdf extracted into files with extension pbm or ppm
mogrify -format png -resize 50% *.pbm
This will resize the pbm files by 50% and convert them to png. Then you can assemble the png files into a pdf again. You can install gscan2pdf for this.