Update an Existing PDF Document

Hi Group,

would someone please direct me to a Rails GEM | Plugin that would
enable me to update an existing PDF document.

I am attempting to complete a PDF document (actually a HM Revenue &
Customs form) from data in a MySQL database.

All that I have seen so far are examples of generating a PDF document
from scratch. Is there anything out there that would enable me to
open an existing document and maybe update fields in the PDF document?

Thanks in advance.

Kind Regards