You can (and should) resolve this by removing that
dompdf.php file from your system. It sounds like the previous version codebase was not removed during your most recent upgrade.
CIVI-SA-2015-001 was a security release which removed
dompdf.php from the CiviCRM installation, as it permitted attackers to read arbitrary files from your webserver. This permits an attacker to retrieve the contents of files they would not normally be permitted to view - even system configuration files.
CiviCRM upgrade instructions advise to back up and then delete previous version code files before installing the new CiviCRM codebase, since leaving old code files in place may affect the behaviour of CiviCRM (and in cases like this, the removal of a file is the fix required).
A full list of recent CiviCRM security advisories is available at https://civicrm.org/advisory, and CiviCRM site administrators should sign up for security upgrade notifications