https://github.com/mozilla/pdf.js/issues/6397#issuecomment-164954332 Would have been a lot easier to find had there been an assertion checking that string16 didn't go out of bounds of the 16bit number.

if (!properties.fontMatrix) {
    return;
  }
  if (properties.fontMatrix[0] === FONT_IDENTITY_MATRIX[0]) {
    return;
  }
  
  const scale = 0.001 / properties.fontMatrix[0];
  const glyphsWidths = properties.widths;
  for (const glyph in glyphsWidths) {
    glyphsWidths[glyph] *= scale;
  }
  properties.defaultWidth *= scale;
}
function adjustToUnicode(properties, builtInEncoding) {