Terminal Lucidity is completely free! The DLC is to support the developer if you enjoyed the experience.

As a token of gratitude, this comes with a short digital game manual (art book) with some developer commentary.

Game Manual File Path: (Installation Path)\Terminal Lucidity Game Manual.pdf
Ex: C:\Program Files (x86)\Steam\steamapps\common\Terminal Lucidity\Terminal Lucidity Game Manual.pdf

Or right-click the game in the Steam Library, select properties, navigate to installed files, then click on "Browse..."