Go through all document content-types (should be PDFs), generate a thumbnail of the first page of the PDF and save the thumbnail in a field of the document.

thumbails.php

<?php

// Select all nodes of type Document
$result = db_select('node', 'n')
  ->fields('n', array('nid'))
  ->condition('type', 'document')
  ->execute();

// create thumbnail and attach it
foreach($result as $row) {
  $node = node_load($row->nid);
  create_thumbnail($node);
}

exit(0);                                                                                                                                                                                  

function create_thumbnail($node) {

  $document = entity_metadata_wrapper('node', $node);
  $attachments = $document->field_attachment->value();
  $first_attachment = $attachments[0];
  $title = $document->title->value();

  $pdf = drupal_realpath($first_attachment['uri']);

  $image_name = sprintf("doc-thumbnail-%05d.png", $node->nid);  
  $dest_uri = 'public://document/thumbnails/tmp/' . $image_name;
  $image_path = drupal_realpath($dest_uri);

  // run convert
  $cmd = sprintf("/usr/bin/convert '%s'[0] -density 150x150 %s",
    $pdf,
    $image_path
  );  
  system($cmd);

  echo "Executing: [$cmd]\n";

  $file = (object) array(
      'uid' => 1,
      'uri' => $dest_uri, 
      'filemime' => file_get_mimetype($image_path),
      'status' => 1,
      'title' => $title,
      'alt' => $title,
  );
  
  $file = file_copy($file, 'public://document/thumbnails/' . $image_name, FILE_EXISTS_REPLACE);
  if ($file) {
    // save node
    $document->field_image = array('fid' => $file->fid);
    $document->save();
  }
  else {
    echo "Problems generating thumbnail for [$node->title] (nid: $node->nid)\n";
  }
}