Check out LibreOfficeKit

LibreOfficeKit is getting more and more powerful. The whole LibreOffice-Android app is build on top of LibreOfficeKit. Ubuntu's Documents app seems to be using it (https://www.youtube.com/watch?v=Itjxlzr8Bmk).
There is already a Gtk+ docview Widget (http://www.ahunt.org/2014/06/libreofficekit-gtk-viewer-widget/)
Also the im/exporter for ms office are gettting better and better.
Here is a lengthy blogpost
https://people.gnome.org/~michael/blog/2015-08-05-under-the-hood-5-0.html

edit1:
another blog post about the gtk3 widget and integration in gnome-documents
http://pranavk.github.io/open-source/gsoc-summer-wrapup-report/
https://wiki.documentfoundation.org/Development/Integrating_LOKDocView_and_GNOME_Documents