If you create a PDF from an image it’ll usually be huge because of the image quality. To considerably reduce its size you can use the gs tool.

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=new.pdf original.pdf