Zena,
Thank you for your question.
At the moment you can only digitally sign text documents. BE text documents already have digital signature code added.
If you need to add digital signing to custom text document, please add liquid code. This case explains how to do it: CASE: implement digital signatures into templates
To sign documents digitally, you need to set up Ok!Sign integration.
Information how to do it: https://en.help.silverfin.com/category/509-integrations
Please let us know if you need any additional information.
Jelena