As mentioned at the top of that page, for full text searching (& thumbnails) to work, the "media-filter" tool must be run on a regular basis. This is the tool that generates both full text extraction and thumbnails. If you are just demoing DSpace, you can run this tool manually from the Admin UI (see the Processes menu). But, for a production-level site, it's better to schedule this tool to run regularly (via a cron job)...usually either once a day, or possibly a few times a day (if you receive a lot of new content in one day). More information on this tool is in the docs at
https://wiki.lyrasis.org/display/DSDOC7x/Mediafilters+for+Transforming+DSpace+Content