Attach (recommended) or Link to PDF file here:
[test.pdf](https://github.com/mozilla/pdf.js/files/6270827/test.pdf)
Configuration:
- Web browser and its version:chrome 89
- Operating system and its version:win10
- PDF.js version: 2.4/2.7/2.8 no working
- Is a browser extension: no 

What is the expected behavior? (add screenshot)
![image](https://user-images.githubusercontent.com/18545199/113852392-1ae26080-97cf-11eb-9dc9-0cecf52d267b.png)

What went wrong? (add screenshot)
![1617791294(1)](https://user-images.githubusercontent.com/18545199/113852355-10c06200-97cf-11eb-8672-598750f1093a.jpg)


console this log : **FormatError: TrueType Collection does not contain "DCWGQU+CambriaMath" font** 

any solution?thanks
}
      
      return (charsCache[charsCacheKey] = glyphs);
    },
    /**
     * Chars can have different sizes (depends on the encoding).
     * @param {String} a string encoded with font encoding.
     * @returns {Array<Array<number>>} the positions of each char in the string.
     */
    getCharPositions(chars) {
      
      
      const positions = [];
      if (this.cMap) {
        const c = Object.create(null);
        let i = 0;
        while (i < chars.length) {
          this.cMap.readCharCode(chars, i, c);
          const length = c.length;
          positions.push([i, i + length]);
          i += length;
        }
      } else {
        for (let i = 0, ii = chars.length; i < ii; ++i) {
          positions.push([i, i + 1]);
        }
      }
      return positions;
    },
    get glyphCacheValues() {