Spotlight receives the name, the first 16 KB of text content, the comment, and the tags for each item as well as metadata like the item’s type, its icon, and the creation and modification dates.
16 KB seems to be the internal limit of Spotlight (as of today).
Check DEVONthink To Go’s settings in the Settings app and make sure that Search & Siri Suggestions is switched on. Check also DEVONthink To Go’s own settings and switch on Spotlight if necessary. Then try to search for terms that you know that occur in the document, e.g. in its title. If still no results show up rebuild the Spotlight index (see the build-in documentation, chapter Appendix).