How to put watermark on PDF with digital signature?

Question

PHP

Guys, hello!
Urgently need help!!!
Who faced such a task, which is described in the question?
I use domPDF to generate PDF, but this is one thing, I can watermark such a PDF file using the FDPF and FPDI libs.
But the customer has the task of putting a watermark also on a protected file with a digital signature.
The whole project has already been filed, everything is ready, I started to give up, and then a jamb surfaced that FPDI (free version) cannot put a watermark on a file with an EDS, because. it is compressed.
Guys, I urgently need help, my hair has already sat with this crap, please help. Sketch options for solving this problem and plus links, if possible! We need free solutions, because. the customer refuses to pay additional costs for the purchase of a library that can work with PDF, on which EDS.

Answer 1

EMNIP, when adding something to a file signed with an EDS, the signature becomes invalid. So watermarks must be added before signing.