https://askubuntu.com/questions/113544/how-can-i-reduce-the-file-size-of-a-scanned-pdf-file

pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf
convert file1.pdf file2.pdf merged.pdf
convert -rotate 90 page\*.jpg +compress file.pdf
convert -density 200x200 -quality 60 -compress jpeg input.pdf output.pdf