title: How to crop PDF margins using pdftk and /MediaBox
body: <p>I used <code>pdftk</code> to uncompress a PDF and then opened it as a text file.<br>
  I want to edit the <strong>/MediaBox</strong> field, which is in my case</p>
  
  <pre><code>/MediaBox [0 0 612 792]
  </code></pre>
  
  <p>I would like to reduce the margins, for instance </p>
  
  <pre><code>/MediaBox [100 0 512 792]
  </code></pre>
  
  <p>Unfortunately it doesn't work. I can change the <code>0</code> into a <code>2</code> or a <code>9</code> but I cannot put <code>100</code> for instance.</p>
  
  <p>Any idea why?</p>