How secure is this PDO Portfolio Script?
Your database queries are using bound parameters with prepared statements. Good.
You need to escape your output.


Other observations:


Your Database class adds nothing (unless you've abbreviated it here for simplicity?). Consider simply using PDO.
You have no error handling. Try loading your page with incorrect db credentials and see what I mean.
Consider using markup with php instead of string-ified markup in php. It is much easier to read and maintain.