Lots of software can manipulate PDF. Open PDF in libredraw change pages,print as PDF or export as PDF. A system that skims content is purposely going to bypass any signed restriction.

Edit: Here’s how to bypass restriction in Paperless OCR.

The parameter PAPERLESS_OCR_USER_ARGS: ‘{“invalidate_digital_signatures”: true}’ in the context of Paperless-ngx and OCRmyPDF allows OCR processing of PDF documents that have been digitally signed by intentionally invalidating those signatures. In its standard configuration, OCRmyPDF does not process documents with digital signatures so as not to compromise their integrity. Setting this parameter to true allows OCR on such documents