How to sign a pdf document

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In today's communication, it is important to make sure that documents that have been sent over the internet, for example, via mail arrive unchanged and that the receiver can check that the document has been sent by the person claiming to be the sender.

The public key infrastructure (PKI) of the Deutsche Forschungsnetz (DFN) allows you as a researcher, student or staff member to generate a certificate that you can use to sign emails or documents. You can request your certificate from your local certificate authority (usually the IT-center of the university or research institution). You can find a list if local certificate authorities at For members of Paderborn University, the corresponding certificate authority is the Zentrum fuer Informations- und Medientechnologien (IMT). You can find instructions on how to request a certificate at

Once you have obtained a certificate you can use it to

We as PC2 trust your digital signature because you had to authenticate at your local certificate authority to obtain the certificate. Thus, using digital signatures you can avoid printing the document, signing it by hand and bringing it to us or sending it via mail.

Signing pdfs can be achieved with several programs:

  • Adobe Reader (see
  • Libreoffice
    • If you store your certificate in the certificate store of Firefox or Thunderbird, you can also use the certificate in Libreoffice.
    • To sign a document, open Libreoffice and open "File"->"Digital Signatures"->"Digital Signatures...". Then click "Sign Document". You will be asked for your the Master password of your Firefox/Thunderbird certificate database (NSS certificate DB). Then you can select your certificate and sign the document.
    • To sign an existing pdf, you can use "File"->"Digital Signatures"->"Sign Existing PDF" and select the pdf you want to sign. This will open the pdf as a read-only document in Libreoffice. You can now sign it by clicking "Sign Document" in the upper right corner.
  • To check the signature you can
    • open the pdf document again with Libreoffice, Adobe Reader, or any other pdf viewer that support sigantures,
    • or under Linux you can check the signature with the command line tool "pdfsig" (from poppler-utils).