Nobody asked, but for the record, this is how make real one-page PDF from two-page scans. (gm = GraphicsMagick)

    mkdir kaksi
    rm kaksi/*
    j=102 
    scale=600
    size=500x730
    yla=27
    for x in *.pnm ; do
     echo $x
    gm convert $x -rotate 90 -crop $size+20+$yla -resize $scale -normalize kaksi/k$j.jpg
      j=$((j+2))
    done    
    j=103
    for x in *.pnm ; do
     echo $x
    gm convert $x -rotate 90 -crop $size+530+$yla -resize $scale -normalize kaksi/k$j.jpg
      j=$((j+2))
    done
    cd kaksi
    gm convert *.jpg -format pdf TheBook.pdf