ONLYOFFICE Desktop Editors is a free open-source office suite (much like LibreOffice) that contains viewers and editors for text documents, spreadsheets, and presentations. Along with offline work, it’s possible to connect the app to the cloud (ONLYOFFICE, Nextcloud, ownCloud, Seafile) and collaborate on docs online. The code repository is available on GitHub under AGPL v.3.0 license.
LibreOffice has similar functionality and interesting to note by them that whilst ODF format is fully signed including the metadata, this is not the case for OOXML (docx) as they have to disable the signing for metadata as Microsoft does not sign the metadata (I wonder why?).
See short tutorial at How to protect documents with a digital signature in ONLYOFFICE Desktop Editors v.6.2
In this tutorial, we’ll learn how to use a digital signature in ONLYOFFICE Desktop Editors to protect your documents. ONLYOFFICE Desktop Editors is ...