Text Content Viewer uses the keyword search index that may have been populated during Image Ingest. If a file has text stored in the index, then this tab will be enabled and it will be displayed to the user.
This tab may have more text on it than the "Strings Content Viewer", which relies on searching the file for text-looking data. Some files, like PDF, will not have text-looking data at the byte-level, but the keyword indexing process knows how to interpret a PDF file and produce text.
If this tab is not enabled, then either the file has no text or you did not enable Keyword Search as an ingest module. Note that this viewer is also used to display keyword hits.
