If a pdf contains pictures (jpeg, jpg), the final pdf file can be very big. Usually people have some problems sending or receiving this huge files in their emails. To reduce this size and still get a pdf that has printing quality use:
This will produce a pdf file out of the input pdf file with an extra pdf suffix. This file will be much smaller. If this is not enough, then you will probably have to generate the pdf file again but with lower resolution pictures. For this you can use convert:
in_file="huge_picture.jpeg" out_file="small_filesize_picture.jpeg" convert -page a4 -resample %30 $in_file $out_file
You can use convert also to convert to other picture formats at the same time.