A DAT viewer displays the text data in a DAT. This can be displayed in 2 ways, in raw text for standard format text or in a spreadsheet format when the DAT holds table formatted text.
If the DAT is editable, the text can be edited directly in the DAT viewer. Editable text is displayed in blue. Non-editable text is displayed in purple, DATs which get their data via the node's input are non-editable.
Ctrl+g will reset the text to the Preferred Text Size set in Preferences. Ctrl+h will attempt to fit the entire body of text in the available viewer size. Scaling down text size to fit the viewer is limited by the Minimum Text Size set in Preferences. If the text is non-editable, g and h can be used without holding down the ctrl key.