Well that error indicates the class you are calling getPdf() on doesn’t exist. If you check out appcodecoreMageCoreModelConfig.php (~ line 1352 (as of version 1.9.4.5)) specifically the getModelInstance() function, you can see that it only returns a boolean if the class doesn’t exist or if there is a problem including the file (in the new $className area). If the problem is in the new $className area it might be helpful to remote the @ in the autoloader (libVarienAutoload.php ~ line 94) so you can see some errors in your logs.
