Hello I want to convert my doc file to pdf. In this code I changed newfile.docx to newfile.pdf but it won't work. How do I save the file as pdf?

$x = "Hello World!";

include_once('includes/tbszip.php');

$zip = new clsTbsZip();

// Open the document
$zip->Open('mydoc.docx');
$content = $zip->FileRead('word/document.xml');
$p = strpos($content, '</w:body>');
if ($p===false) exit("Tag </w:body> not found in document.");

// Add the text at the end
$content = substr_replace($content, '<w:p><w:r><w:t>'.$x.'</w:t></w:r></w:p>', $p, 0);
$zip->FileReplace('word/document.xml', $content, TBSZIP_STRING);

// Save as a new file
$zip->Flush(TBSZIP_FILE, 'newfile.pdf');