https://forms.enablementadobe.com/content/forms/af/from1040.html fill in some details, then click on the View PDF and then on View PDF check box to see the pdf populated with your data

what is the version of AEM Forms? is this a custom endpoint?

Did you stop the server before creating the install folder and copying the far file?

I have written a tutorial on saving and prefilling forms with data and attachments in the Azure portal. This approach is only supported for forms based on Core ComponentsLet us know if you have any questions on this tutorial

If you are using AEM forms6.5, you will have access to assembler service. you will need to be familiar with assembler service and creating DDX documents https://helpx.adobe.com/pdf/aem-forms/6-2/ddxRef.pdf 

Have you looked at watermarking capabilities using assembler services in AEM forms?

Try passing in the xdp template instead of the pdf

try Passing in the xdp template to generatepdfoutput method

how are you updating the fields in the new form?

not necessary to use workflow. The article I sent was for 6.5