What happened to vTiger CRM --the fork of SugarCRM-- after the upgrade to versions 5.2x? Among many good and improved things, the PDF generating libraries have changed paths. Some plug and play, or click and point, PDF making extensions, crafted for 5.1x won't work with the new versions 5.2x -- until they will be upgraded at their turn, of course.

In the while, for the code-minded people, here's the new list of

paths where you may reach and edit libraries to make your PDF Invoices and Quotes, Sales Orders, etc. as custom as you wish. Because any company would want a unique type looking design for its documents.

include/InventoryPDFController.php
include/InventoryPDF.php
include/tcpdf/template/body.php
include/utils/CommonUtils.php

modules/Invoice/InvoicePDFController.php
modules/Invoice/pdf_templates/footer.php
modules/Invoice/pdf_templates/header.php
modules/Invoice/pdf_templates/lastpage/body.php
modules/Invoice/pdf_templates/lastpage/footer.php

modules/Quotes/QuotePDFController.php
modules/Quotes/pdf_templates/footer.php
modules/Quotes/pdf_templates/header.php
modules/Quotes/pdf_templates/lastpage/body.php
modules/Quotes/pdf_templates/lastpage/footer.php

modules/SalesOrder/SalesOrderPDFController.php
modules/SalesOrder/SalesOrderPDFHeaderViewer.php
modules/SalesOrder/pdf_templates/footer.php
modules/SalesOrder/pdf_templates/header.php
modules/SalesOrder/pdf_templates/lastpage/body.php
modules/SalesOrder/pdf_templates/lastpage/footer.php

vtlib/Vtiger/PDF/PDFGenerator.php
vtlib/Vtiger/PDF/TCPDF.php
vtlib/Vtiger/PDF/inventory/ContentViewer2.php
vtlib/Vtiger/PDF/inventory/ContentViewer.php
vtlib/Vtiger/PDF/inventory/FooterViewer.php
vtlib/Vtiger/PDF/inventory/HeaderViewer.php
vtlib/Vtiger/PDF/models/Model.php
vtlib/Vtiger/PDF/viewers/ContentViewer.php
vtlib/Vtiger/PDF/viewers/FooterViewer.php
vtlib/Vtiger/PDF/viewers/HeaderViewer.php
vtlib/Vtiger/PDF/viewers/PagerViewer.php

Have fun! And don't forget to backup first!
BTW, in the case you consider that you can spend your time in a better way than code-guessing and fixing, it's always a good idea to give us a line and we're gonna handle that for you. According to the specs.