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.

Microsoft Edge:

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.