In file https://github.com/mozilla/pdf.js/files/5249872/product1.pdf, the fifth text widget has a default appearance using font F1 and this one is neither in DR nor in AcroForm but in AP::N::Resources.
if (fontSize === null || fontSize === 0) {
      let capHeight;
      if (font.capHeight) {
        capHeight = font.capHeight;
      } else {
        const glyphs = font.charsToGlyphs(font.encodeString("M").join(""));
        if (glyphs.length === 1 && glyphs[0].width) {
          const em = glyphs[0].width / 1000;
          
          
          capHeight = 0.7 * em;
        } else {
          capHeight = 0.7;
        }
      }
      
      fontSize = Math.max(1, Math.floor(height / (1.5 * capHeight)));
      let fontRegex = new RegExp(`/${fontName}\\s+[0-9.]+\\s+Tf`);
      if (this.data.defaultAppearance.search(fontRegex) === -1) {
        
        fontRegex = new RegExp(`/${fontName}\\s+Tf`);
      }
      this.data.defaultAppearance = this.data.defaultAppearance.replace(
        fontRegex,
        `/${fontName} ${fontSize} Tf`
      );
    }
    return fontSize;
  }
  _renderText(text, font, fontSize, totalWidth, alignment, hPadding, vPadding) {