Using the new interface Document.GetPageLabel(int pageno) or the Advanced properties, you can get the custom page labels from pdf catalog, check the below code:

Note: a premium license is needed for advanced properties.

Based on PDF reference 1.7:

A document’s labeling ranges are defined by the PageLabels entry in the document catalog (see Section 3.6.1, “Document Catalog”). The value of this entry is a number tree (Section 3.8.6, “Number Trees”), each of whose keys is the page index of the first page in a labeling range. The corresponding value is a page label dictionary defining the labeling characteristics for the pages in that range. The tree must include a value for page index 0. Table 8.10 shows the contents of a page label dictionary. (See implementation note 76 in Appendix H.)

Example 8.5 shows a document with pages labeled
i, ii, iii, iv, 1, 2, 3, A−8, A−9, …
Example 8.5
1 0 obj
  << /Type /Catalog
     /PageLabels << /Nums [ 0 << /S /r >>% A number tree containing
                            4 << /S /D >>% three page label dictionaries
                            7 << /S /D
                                 /P ( A− )
                                 /St 8
                              >>
                          ]
                  >>
    …
  >>
endobj

/**
 * get label of page
 * @param pageno 0 based page index number
 * @return json string or pure text. for json: name is style name of number.<br/>
 * for example:<br/>
 * {"D":2} is "2"<br/>
 * {"R":3} is "III"<br/>
 * {"r":4} is "iv"<br/>
 * {"A":5} is "E"<br/>
 * {"a":6} is "f"<br/>
 * for pure text: the text is the label.
 */
public String GetPageLabel(int pageno)
{
   return getPageLabel(hand_val, pageno);
}

 

Or using advanced properties :

Ref ref = doc.Advance_GetRef();
if(ref != null) {
    Obj rootObj = doc.Advance_GetObj(ref);
    if(rootObj != null) {
        int count = rootObj.DictGetItemCount();
        for(int cur = 0; cur < count; cur++) {
            String tag = rootObj.DictGetItemTag(cur);
            Obj item = rootObj.DictGetItem(cur);
            if(tag.equals("PageLabels") && item.GetType() == 8) {
                rootObj = doc.Advance_GetObj(item.GetReference());
                if(rootObj.DictGetItemCount() > 0 && rootObj.DictGetItem(0).GetType() == 6) {
                    item = rootObj.DictGetItem(0);
                    int arrayCount = item.ArrayGetItemCount();
                    for(int i = 0; i < arrayCount; i++) { //note from here it can be handled based on (Table 8.10 - Entries in a page label dictionary) in PDF reference 1.7
                        rootObj = item.ArrayGetItem(i);
                        if(rootObj.GetType() == 2)
                            Log.i("--ADV--", "array item " + i + ": value = " + rootObj.GetInt());
                        else if (rootObj.GetType() == 7)
                            handleDictionary(rootObj, doc);
                        else if(rootObj.GetType() == 8) {
                            rootObj = doc.Advance_GetObj(rootObj.GetReference());
                            if (rootObj.GetType() == 7)
                                handleDictionary(rootObj, doc);
                        }
                    }
                }
                break;
            }
        }
    }
}

---

private  void handleDictionary(Obj obj, Document doc) {
    try {
        int count = obj.DictGetItemCount();
        for (int cur = 0; cur < count; cur++) {
            String tag = obj.DictGetItemTag(cur);
            Obj item = obj.DictGetItem(cur);
            int type = item.GetType();
            String type_name = get_type_name(type);

            Log.i("--ADV--", "tag:" + cur + "---" + tag + ":" + type_name + " ->");

            if (type == 1) //boolean
                Log.i("--ADV--", " value = " + item.GetBoolean());
            else if (type == 2) //int
                Log.i("--ADV--", " value = " + item.GetInt());
            else if (type == 3) //real
                Log.i("--ADV--", " value = " + item.GetReal());
            else if (type == 4) //string
                Log.i("--ADV--", " value = " + item.GetTextString());
            else if (type == 5) //name
                Log.i("--ADV--", " value = " + item.GetName());
            else if (type == 6) { //array
                int arraycount = item.ArrayGetItemCount();
                for (int k = 0; k < arraycount; k++) {
                    Obj array_obj = item.ArrayGetItem(k);
                    Log.i("--ADV--", "array item " + k + ": value = " + array_obj.GetReal());
                }
            } else if (type == 7) //dictionary
                handleDictionary(item, doc);
            else if(type == 8) {
                handleDictionary(doc.Advance_GetObj(item.GetReference()), doc);
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}