Thanks for your message. We understand what you’re looking for, and our documentation does not say much about the customization for the default PDF documents, especially because this is something pretty technical that requires to be familiar with PHP coding.
Adding the generation of a new custom PDF document would require changes to be made on core files, and so this is strongly discouraged, especially because by installing any future update for our plugin all of your changes would go lost. Instead, if you are looking to customize the existing PDF document for the contract/agreement that gets automatically generated whenever a rental order gets confirmed (maybe after a successful payment), then you can use the apposite tool in the Configuration page to edit the template file for the contract. Our suggestion in this case is to debug the content of the available variables, objects and arrays directly in the source code of the template file.
We invite you to get in touch with our team through our websites should you have any technical questions.
Thank you,
The VikWP Team