Version 4.7.3: TCPDF package seems to be missing
In 4.7.3 the TCPDF package was moved to civicrm/vendor/tecnickcom  from civicrm//packages/tcpdf.

I just tested on a WP 4.5.2 and CiviCRM 4.7.8 install and was able to create mailing labels.

Can you give us the steps to reproduce?  This may be a bug that needs an issue filed at https://issues.civicrm.org

One additional thought - try and empty the caches in WP the url is https://yourdomain.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm/admin/setting/updateConfigBackend&amp;reset=1