Convert or split scanned pdf pages one into two

Many people might have scanned books which has two book pages per page. If you are reading them on a pad, it is not very convenient. Below are the steps to convert them.

1. Extract the pictures from the pdf file
   

   pdfimage some.pdf book
   

   It is better to use pdfimage than to do
   

   convert some.pdf some.jpg
   

2. View with a picture viewer to determine the area you want to crop.
3. Then run the following bash scripts
   

   #!/usr/bin/env bash
   page=0
   for i in *pbm
   do
       let page=page+1
       #crop dimensions and offsets
       convert $i -crop 1850x1200+30+30 $page.jpg
       let page=page+1
       convert $i -crop 1850x1200+30+1400 $page.jpg
   done
   #
   # you may also want to adjust the page numbers, e.g.
   #for i in `seq 111 -2 1`
   #do
   #    let j=$i+2
   #    mv $i.jpg $j.jpg
   #done
   #
   # let the jpg files have the same name length
   # usually a book has less than 1000 pages
   for i in ?.jpg
   do
       mv $i 00$i
   done
   
   for i in ??.jpg
   do
       mv $i 0$i
   done
   
   convert *jpg tgt.pdf
   

File tgt.pdf is what you want.
Note,

1. pbm is for monochrome images, ppm is for non-monochrome images.
2. change to png format to reduce the pdf file size if it is black and white.