Attach (recommended) or Link to PDF file here:
http://help.adobe.com/en_US/livecycle/11.0/SampleForms/941/Form.pdf

- Web browser and its version: Firefox 89.0.1
- Operating system and its version: Windows 10 10.0.19043.1052
- PDF.js version: commit bdfcdd6f8326f22c135edfa29a341ea4c558f3d1
- Is a browser extension: No

Steps to reproduce the problem:
1. Enable XFA support
2. Open form

What is the expected behavior? (add screenshot)
Page 3 (of 4) renders
![image](https://user-images.githubusercontent.com/7104454/122686842-622e9980-d1e1-11eb-92ab-86fe488560f2.png)

What went wrong? (add screenshot)
Only 3 pages total render in pdf.js
while (!found && pos > 0) {
        pos -= step - startXRefLength;
        if (pos < 0) {
          pos = 0;
        }
        stream.pos = pos;
        found = find(stream, STARTXREF_SIGNATURE, step, true);
      }
      if (found) {
        stream.skip(9);
        let ch;
        do {
          ch = stream.getByte();
        } while (isWhiteSpace(ch));
        let str = "";
        while (ch >=  0x20 && ch <=  0x39) {
          str += String.fromCharCode(ch);
          ch = stream.getByte();
        }
        startXRef = parseInt(str, 10);
        if (isNaN(startXRef)) {
          startXRef = 0;
        }
      }
    }
    return shadow(this, "startXRef", startXRef);
  }
  
  
  checkHeader() {