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 if a file or a result associated with a file is selected.
This tab may have more text on it than the "String View", 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. For the files the indexer knows about, there may be the METADATA section at the end of the displayed extracted text. If an indexed document contains any metadata (such as creation date, author, etc), it will be displayed there. Note that, unlike the "String View", the Text View does not have its built-in settings for the script/language to use for extracted strings. This is because the script/language is used at indexing time, and that setting is associated with the Keyword Search indexer, not the viewer.
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 highlighted keyword hits when operated in the "Search Matches" mode, selected on the right-hand side of the viewer's toolbar.
