Example of adding files to solr document that are only linked to in the node body.

example.php

<?php

/**
 * Implements hook_apachesolr_document_handlers().
 */
function example_apachesolr_document_handlers($type, $namespace) {
  if ($type == 'node' && $namespace == 'example') {
    return array('example_add_documents');
  }
}

/**
 * Implements hook_apachesolr_update_index().
 *
 * @see apachesolr_attachments_update_index().
 */
function example_apachesolr_update_index($document, $node, $namespace) {
  if (variable_get('apachesolr_attachments_extract_using', 'tika') == 'solr' || variable_get('apachesolr_attachments_tika_path', '')) {
    $start = REQUEST_TIME;
    $cron_try = variable_get('apachesolr_attachments_cron_try', 20);
    $cron_limit = variable_get('apachesolr_attachments_cron_limit', 100);
    $cron_time_limit = variable_get('apachesolr_attachments_cron_time_limit', 15);
    $num_tried = 0;
    do {
      $rows = apachesolr_get_nodes_to_index('example', $cron_try);
      // Calls apachesolr_attachments_apachesolr_document_handlers() and
      // ultimately apachesolr_attachments_add_documents().
      $success = apachesolr_index_nodes($rows, 'example');
      $num_tried += $cron_try;
    } while ($success && ($num_tried < $cron_limit) && (REQUEST_TIME - $start < $cron_time_limit));
  }
}

/**
 * This assumes the files you are indexing are not part of the
 * files table but rather just linked in the node body.
 */
function example_add_documents($node, $namespace) {
  $hash = apachesolr_site_hash();
  // Find all of the documents.
  $documents = $matches = array();
  if (preg_match_all('/<a ([^>]+)?href="([^"]+\.(pdf|txt))"/', $node->body, $matches)) {
    foreach ($matches[2] as $href) {
      $file = pathinfo($href);
      $text = apachesolr_attachments_get_attachment_text($file);
      if (!$text) {
        watchdog('example', "No text to index, bro!");
        continue;
      }

      // Create the solr document.
      $document = new ApacheSolrDocument();
      // A single file might be attached to multiple nodes.
      // You might want to use something other than the filename
      // (if it's not unique) to define the document_id, a separate
      // table to track these files could work, or something like UUID.
      $file_id = $file['basename'];
      $document->id = apachesolr_document_id($file_id . '-' . $node->nid, 'file');
      $document->site = url(NULL, array('absolute' => TRUE));
      $document->hash = $hash;
      $document->entity_type = 'file';
      $document->entity_id = $file_id;
      $document->bundle = $node->type;
      $document->bundle_name = node_type_get_name($node);
      $document->label = $file['filename'];
      $document->is_nid = $node->nid;
      $document->url = file_create_url($href);
      $document->path = file_stream_wrapper_get_instance_by_uri($href)->getDirectoryPath() . '/' . file_uri_target($href);

      $document->content = $file['filename'] . ' ' . $text;

      $document->ss_name = $node->name;
      // We want the name to ale be searchable for keywords.
      $document->tos_name = $node->name;

      // Everything else uses dynamic fields
      $document->is_uid = $node->uid;
      $document->bs_status = $node->status;
      $document->bs_sticky = $node->sticky;
      $document->bs_promote = $node->promote;
      $document->is_tnid = $node->tnid;
      $document->bs_translate = $node->translate;
      if (empty($node->language)) {
        // 'und' is the language-neutral code in Drupal 7.
        $document->ss_language = LANGUAGE_NONE;
      }
      else {
        $document->ss_language = $node->language;
      }
      $document->ds_created = apachesolr_date_iso(filectime($href));
      $document->ds_changed = $document->ds_created;

      // apachesolr_attachments-specific fields.
      $document->ss_filemime = file_get_mimetype($href);
      $document->ss_file_node_title = apachesolr_clean_text($node->title);
      $document->ss_file_node_url = url('node/' . $node->nid, array('absolute' => TRUE));

      // Add taxonomy to document.
      $indexed_fields = apachesolr_entity_fields('node');
      foreach ($indexed_fields as $index_key => $field_info) {
        if ($field_info['field']['type'] == 'taxonomy_term_reference') { // Add only taxonomy.
          $field_name = $field_info['field']['field_name'];
          // See if the node has fields that can be indexed
          if (isset($node->{$field_name})) {
            // Got a field.
            $function = $field_info['indexing_callback'];
            if ($function && function_exists($function)) {
              // NOTE: This function should always return an array.  One
              // node field may be indexed to multiple Solr fields.
              $fields = $function($node, $field_name, $index_key, $field_info);
              foreach ($fields as $field) {
                // It's fine to use this method also for single value fields.
                $document->setMultiValue($field['key'], $field['value']);
              }
            }
          }
        }
      }


      // Let modules add to the document.
      foreach (module_implements('apachesolr_update_index') as $module) {
        $function = $module . '_apachesolr_update_index';
        $function($document, $node, $namespace);
      }
      drupal_alter('apachesolr_attachments_index', $document, $node, $file, $namespace);

      $documents[] = $document;
    }
  }

  return $documents;
}