Drupal - a very popular open source CMS (Content Management System) - doesn't have a built-in support for mobile and handheld devices (PDAs). If you want to make your site more accessible to mobile users, your only option now is to have an additional "mobile" theme in your set-up, which would be very simple and lean, and somehow use it automatically when users access your site from small devices. Since automatic detection is required and you probably don't want to change core Drupal files, you can use this module Mobile Theme - it handles the selection for you. It works pretty well, but please be aware that it also requires the Browscap module installed.
Once done everything works pretty smoothly. In your theme configuration page of Drupal administrative pages you will find a drop-down box, which allows to select the "mobile" theme for your site. It will be used automatically when you access it from such a device.
A number of mobile themes for Drupal exist. You can consider one of these: