Gets the horizontal scale applied in text space so that a glyph drawn with a substitute fills the width the PDF gives it: the font's width for the code divided by the substitute's width for the glyph. 1 for a glyph drawn with the font's own program.
public double HorizontalScale { get; }
ISO 32000-2 §9.2.4 and §9.6.2.2: the glyph is positioned by the font's widths whatever program draws it (font substitution). Only horizontal writing is scaled.