So basically the feature is to digitally sign a PDF using a USB token. Digitally signing code is present on Page 96 in this iText guide. Also, this iText repo.

How can I setup an open source USB token on Mac OS for testing. Would I have to buy any of the USB token which are compatible with OpenSC mentioned here?

Below link assumes that .so file is already present. https://developers.yubico.com/YubiHSM2/Usage_Guides/OpenSSL_with_pkcs11_engine.html