Yes I'm not using composer.
I also have tried the require_once thing and its still giving same error.

 
<?php
	require_once(BASE_PATH.'/tcpdf/thirdparty/tcpdf/tcpdf.php'); 
	class RaaOrders Extends ReportsModule {
 

Instead of giving you so much trouble, let me first try using composer then if it don't work I will get back to you please.

@swaiba, thank you very much for suggesting composer will help me solve the issue. Can now generate pdf using the code in my function pdf(). Now the problem is getting it to render content that is in my .ss file to the pdf. How would I do that?

This would achieve that..

$pdf = new TCPDF();
$pdf->AddPage();
$vd = new ViewableData();
$str = $vd->customise(ArrayData::create(array('MyVariable'=>'MyValue')))->renderWith('MyTemplateName');
$pdf->writeHTMLCell(0, 0, '', '', $str, 0, 1, 0, true, '', true);
$strContent = $pdf->Output(BASE_PATH.'/tmp/tmp.pdf', 'S');

@Swaiba, everything is now working like a charm. I very much appreciate the help