An illustration of how to manipulate the metadata or data used to generate the PDF.     

function redcap_pdf($project_id, $metadata, $data, $instrument=null, $record=null, $event_id=null, $instance=1)
{
    // Loop through $metadata and alter a field attribute
    foreach ($metadata as &$attr) {
        // Replace all labels with static text (proof of concept)
        $attr['element_label'] = "This is a field label.";
    }

    // Remove a specific record from the $data array (record "101" in this example)
    unset( $data['101'] );

    // Return metadata and data in an associative array after being manipulated.
    // If not changed, then no need to return anything.
    return array('metadata'=>$metadata, 'data'=>$data);
}

A much more manageable way to perform project-specific operations for many projects at once is to create a directory structure where each project has its own subdirectory under the main Hooks directory (e.g., named "redcap/hooks/pid{$project_id}/redcap_survey_page.php"). This allows the code for each project to be sandboxed and separated and also makes it more manageable to utilize other files (e.g., PHP, HTML, CSS, and/or JavaScript files) that you can keep in the project's subdirectory (i.e., "pid{$project_id}") in the Hooks folder. Then the designated project handler PHP script can utilize any of the parameters passed in the function to perform actions specific to each project. NOTE: This example assumes that the "hooks" sub-directory is located in the same directory as the PHP file containing the Hook functions.

function redcap_pdf($project_id, $metadata, $data, $instrument=null, $record=null, $event_id=null, $instance=1)
{
    // Set the full path of the project handler PHP script located inside the
    // project-specific sub-folder, which itself exists in the main Hooks folder.
    $project_handler_script = dirname(__FILE__) . "/hooks/pid{$project_id}/".__FUNCTION__.".php";

    // Check if the project handler PHP script exists for this project, and if so,
    // then "include" the script to execute it. If not, do nothing.
    if (file_exists($project_handler_script)) include $project_handler_script;

    // Return metadata and data in an associative array after being manipulated.
    // If not changed, then no need to return anything.
    return array('metadata'=>$metadata, 'data'=>$data);
}