Sorry to insist but have you checked a phpinfo page? Are you sure PDO is not there? Best way is to reinstall php since "when building PDO as a shared extension (not recommended) then all PDO drivers must be loaded after PDO itself."
So installing pdo separately is not really recommended.
Are you on a cpanel vps? Are you using any control panels? Your operating system? Do you have a managed or self managed vps?
If you have cpanel then you have to recompile php using easyapache. You'll find it I think somewhere in WHM ( Apache Update and do your customizations there like adding PDO..if i remember well there is also an advanced options there ). You can also do it running /scripts/easyapache, but I suggest to do it via browser inside WHM.