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