What do you mean by PDO in Drupal and how to enable PDO?

It is an acronym for PHP Data Objects. It is a lean, consistent way to access databases. PDO is just like a data access layer which uses a unified API.

To enable PDO, configure --enable-pdo and --with-pdo-sqlite --with-pdo-mysql or whatever database needs supporting by PDO

For windows users : For Apache, you will need to make sure php_pdo.dll and php_pdo_mysql.dll exist in the php/ext directory, un-comment or add the appropriate lines in php.ini, after that you have to restart the web server. In windows it may no longer be required to enable PDO when using newer versions of PHP, PHP version 5.3 and later. It refers to php_pdo.dll. However, you still need to activate php_pdo_mysql.dll for MySQL or for whichever database you are using.