CakePdf on a php5.6 server - cakephp

Question

The site is on a php5.6 server - shared hosting so installation is done manually using github and php-download.com.
We've chosen to use mpdf and I can create test files by calling it directly.
Our cake site is v3.x.
I've added the pdf extension to routes.php
I've added this to bootstrap.php
Plugin::load('CakePdf',['bootstrap' => true]);
Configure::write('CakePdf', [
'engine' => 'CakePdf.MpdfEngine',
'margin' => [
'bottom' => 15,
'left' => 50,
'right' => 30,
'top' => 45
],
'orientation' => 'portrait',
'download' => true
]);
In my "forms" controller I have 2 functions - they both throw errors on the homepage.
//before the class
use CakePdf\Pdf\CakePdf;
//in the class
function test(){
$this->viewBuilder()->setClassName('CakePdf.Pdf');
$this->viewBuilder()->setLayout('default');
$this->viewBuilder()->options([
'pdfConfig' => [
'engine' => 'CakePdf.MpdfEngine',
'download' => true,
'title' => 'My Form',
'filename' => 'Form1.pdf'
]
]);
}
function test1(){
$pdf_path = WWW_ROOT . 'tmp/tests/'.date('Y-m-d') . DS . 'test1.pdf';
//$CakePdf = new CakePdf();
$CakePdf = new \CakePdf\Pdf\CakePdf\CakePdf();
debug($CakePdf);
exit;
$CakePdf->template('test', 'pdf/default');
$pdf = $CakePdf->output();
$pdf = $CakePdf->write($pdf_path);
}
My error log for /forms/test1 is probably the most useful - in particular the bit where there's the extra 'CakePdf' in the first line
2020-10-12 16:30:12 Error: [Cake\Error\FatalErrorException] Class 'CakePdf\Pdf\CakePdf\CakePdf' not found in /var/www/www.mysite.com/public_html/src/Controller/FormsController.php on line 343
Request URL: /forms/test1.pdf
Stack Trace:
#0 /var/www/www.mysite.com/public_html/vendor/cakephp/cakephp/src/Error/BaseErrorHandler.php(105): Cake\Error\BaseErrorHandler->handleFatalError(1, 'Class 'CakePdf\\...', '/var/www/www.my...', 343)
#1 [internal function]: Cake\Error\BaseErrorHandler->Cake\Error\{closure}()
#2 {main}