The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
} else {
          style = null;
        }
        if (labelDict.has("P")) {
          const p = labelDict.get("P");
          if (!isString(p)) {
            throw new FormatError("Invalid prefix in PageLabel dictionary.");
          }
          prefix = stringToPDFString(p);
        } else {
          prefix = "";
        }
        if (labelDict.has("St")) {
          const st = labelDict.get("St");
          if (!(Number.isInteger(st) && st >= 1)) {
            throw new FormatError("Invalid start in PageLabel dictionary.");
          }
          currentIndex = st;
        } else {
          currentIndex = 1;
        }
      }
      switch (style) {
        case "D":
          currentLabel = currentIndex;
          break;
        case "R":
        case "r":
          currentLabel = toRomanNumerals(currentIndex, style === "r");
          break;