It's possible to sign documents digitally using the Belgian National Identity Card. The official recommendation is “Use Adobe Acrobat on Windows or Mac” however you can sign documents without using proprietary software…

Assuming you have the card, card reader and eID software setup…

• insert eID card into card reader
• open Firefox and go to about:preferences#privacy¹⁾
  • click “Security Devices”
  • in “Device Manager” select “Generic Smart Card Reader interface” (you should see your card info)
  • click “Login” then enter the PIN for your eID card
• open LibreOffice
  • in the “File” menu select “Digital Signatures” and then “Sign Existing PDF“ and chose the file to sign
  • the opened file should have a header containing two buttons “Edit Document” and “Sign Document”
  • click on “Sign Document” (and the next “Sign Document” button than appears…)
  • a pop-up will ask for a password, you will need to enter the PIN for your eID card
  • a “Select Certificate” dialog box should appear.
  • choose the key with your name and “(Authentication)”
  • click “Sign Document” and then “Sign”
  • the “Digital Signatures” dialog should be updated to show that your signature has been added.
  • click “Close”
  • you should then see the document window updated to include signatures
• the PDF should now include your digital signature