Thanks for that - unfortunately, this has left me none the wiser.
The file looks identical to the version I have locally, and I can use it without any problems.
Can you possibly check the file permissions on your server for the lib/tcpdf directory and all subdirectories (and files) - especially lib/tcpdf/fonts/helvetica.php ?
Does the webserver have 'read' permission for all the files within tcpdf (and the subdirectories)? Does it have 'execute' permission for the lib/tcpdf/fonts/ directory?
If you temporarily rename the file lib/tcpdf/fonts/helvetica.php to something else (e.g. helvetica.php.renamed ), does the error message change?
I'm just trying to think of reasons why a file that exists and has the correct content, would produce an error related to it not having the correct content, once it has been included by the PHP code.