I recently upgraded to vesion 4.7.3 (I'm running under Wordpress), and for some reason the TCPDF package was missing!
This led to the error: "Warning: opendir(/nhldata/631/106631/data/web/predigerforum/htdocs/wordpress/wp-content/plugins/civicrm/civicrm//packages/tcpdf/fonts/): failed to open dir: No such file or directory in /nhldata/631/106631/data/web/predigerforum/htdocs/wordpress/wp-content/plugins/civicrm/civicrm/vendor/tecnickcom/tcpdf/tcpdf.php on line 4148 TCPDF ERROR: Could not include font definition file: helvetica"
...when trying to create labels. I copied the directory "tcpdf" from the previous version into the packages directory and it seems to be OK now.
Perhaps there is a slip up in V 4.7.3, because I did nothing more than the normal upgrade process.