2.1.15 [ISO32000-1] Section 9.3.8, Text Knockout
The specification states:
The text knockout parameter, Tk (PDF 1.4), shall be a boolean value that determines what text elements shall be considered elementary objects for purposes of colour compositing in the transparent imaging model.
The parameter value is always taken to be the default, true. That is, all glyphs in the text object shall be treated together as a single elementary object. When glyphs overlap, later glyphs shall overwrite ("knock out") earlier ones in the area of overlap.