Check the following Metalink Note:

Specific Special Characters Are Not Getting Printed In Pdf Output Format From XML Publisher Reports [ID 1283873.1]

The fonts required to publish the special characters may be missing, or some specific patches for the XDOPARSER need to be installed to handle certain special characters.

Check the database and session settings:

SELECT *
FROM   nls_database_parameters

…and…

SELECT *
FROM   nls_session_parameters

…in case a character set is being used which doesn’t contain the problem character (e.g. the pound sign).