Hope this will help you,

In Drupal 7 you can use **[template_preprocess_page][1]** or **[template_preprocess_node][3]** function for your requirement in **template.php** file as **Namari** suggested with [**drupal_add_css**][2].

 - You cant use theme.info file to particular pages. If you add the CSS in theme.info file then the CSS will included for each page. As you want to include the CSS file for particular pages so better to use **template_preprocess_node** OR **template_preprocess_page**.
 
 - **drupal_add_css** : Adds a cascading stylesheet to the stylesheet queue.

<pre><code>
	/**
	 * Implement template_preprocess_page()
	 *
	 */
	function yourThemeName_preprocess_page(&$variables) {
		if (arg(0) == 'node' && is_numeric(arg(1)) && arg(1) == 123) {//check your condition here
		//you can also use $node = $variables['node']; $node is the node object where $node->nid is the node id
			drupal_add_css(drupal_get_path('theme', 'theme_name') . "/cssfilename.css");
		}
	}
</code></pre>

 - You can also use **[template_preprocess_node][3]**
<pre><code>
	/**
	 * Implement template_preprocess_node()
	 *
	 */
	function yourThemeName_preprocess_node(&$vars) {
        $node = $vars['node'];
		if($vars['type'] == 'my-node-type' && $node->nid == 123 ) {// check your condition
				drupal_add_css(drupal_get_path('theme', 'theme_name') . "/cssfilename.css");
		}
	}
</code></pre>

 - If you are checking for a node then better to use **template_preprocess_node** than **template_preprocess_page**


  [1]: https://api.drupal.org/api/drupal/includes!theme.inc/function/template_preprocess_page/7
  [2]: https://api.drupal.org/api/drupal/includes!common.inc/function/drupal_add_css/7
  [3]: https://api.drupal.org/api/drupal/modules!node!node.module/function/template_preprocess_node/7