Sign PDF using a .p12 certificate file

[altered-ego]

As per Q&A session: all you need to know about ONLYOFFICE Docs 8.3 dated 2025/02/10:

@ONLYOFFICE_team

Question: Is it possible to e-sign PDFs with your solution?
Answer: At the moment, digital signatures are available in ONLYOFFICE Desktop Editors for DOCX, XLSX, and PPTX files. We are actively working on implementing the ability to e-sign PDFs as well, and are planning to add this feature in one of the upcoming releases.