It's a bit strange, it wfm on Windows and Mac, I can reproduce the issue on Fedora.
The font used for the second sequence of "x" is a type 1 font and it starts with:
%!PS-AdobeFont-1.1: LucidaTypewriter 1.002
%%CreationDate: 1994 Nov 12 10:47:10
% Bigelow & Holmes, Inc. Pat. Des. 289,422
% Lucida is a registered trademark of Bigelow & Holmes Inc.
11 dict begin
/FontInfo 9 dict dup begin
/version (1.002) readonly def
/Notice (Copyright (c) 1993, 1994 Bigelow & Holmes and Y&Y, Inc. http://www.YandY.com All Rights Reserved.) readonly def
/FullName (Lucida Typewriter) readonly def
/FamilyName (LucidaTypewriter) readonly def
/Weight (Medium) readonly def
/ItalicAngle 0 def
/isFixedPitch true def
/UnderlinePosition -100 def
/UnderlineThickness 50 def
end readonly def
/FontName /URXDVG+LucidaTypewriter-Extend_850 def
/PaintType 0 def
/FontType 1 def
/FontMatrix [0.00085 0 0 0.001 0 0 ] readonly def
In pdf.js, the font is wrapped in a CFF font and we correctly set the matrix transformation:
Out of curiosity, I changed
0.000085 and on Windows I can see a thin "x" but the same height the first ones, but on Fedora nothing I can see is displayed.
Maybe the min of the x-scale and y-scale factors is taken to adjust the font size... I don't know but it's very likely an issue in FreeType.
:jfkthame, maybe you'd know what be could wrong here.