Hi Rokas,
Unfortunately a live preview for the PDF changes is not available at the moment, I’m sorry. The preview is available for the customer email template, but not for the PDF documents as this generation takes up more resources due to the HTML-to-PDF converter class in PHP.
The HTML DOM parser is not a browser, that’s a conversion of HTML markup and CSS rules to PDF instructions, and so building the final PDF file takes up a bit more resources than building a simple HTML content for the email messages.
Our suggestion is to keep a test rental order confirmed in your wp-admin section, and after making changes to the template file, you can choose to re-send the confirmation email and PDF to the client (which should be an administrator or you) from the back-end details page of the rental order. This way you can click the PDF document link on the same page to refresh it and see the changes. This is because browsers may apply cache on PDF files.
We hope this helps!
The VikWP Team