private static Dictionary<IntPtr, byte> processedObjects = new Dictionary<IntPtr, byte>();

private void CloneIndirectObjects(PdfTypeDictionary dict, PdfIndirectList dstList)
{
    foreach (var item in dict)
    {
        if (processedObjects.ContainsKey(item.Value.Handle))
            continue;
        var obj = item.Value;
        processedObjects.Add(obj.Handle, 1);

        if (obj is PdfTypeIndirect)
        {
            obj = (obj as PdfTypeIndirect).Direct.Clone();
            dstList.Add(obj);
            dict.SetIndirectAt(item.Key, dstList, obj);
        }
        if (obj.Is<PdfTypeDictionary>())
            CloneIndirectObjects(obj.As<PdfTypeDictionary>(), dstList);
        else if (obj.Is<PdfTypeArray>())
            CloneIndirectObjects(obj.As<PdfTypeArray>(), dstList);
    }
}

private void CloneIndirectObjects(PdfTypeArray array, PdfIndirectList dstList)
{
    for (int i = 0; i < array.Count; i++)
    {
        if (processedObjects.ContainsKey(array[i].Handle))
            continue;
        var obj = array[i];
        processedObjects.Add(obj.Handle, 1);
        if (obj is PdfTypeIndirect)
        {
            obj = (obj as PdfTypeIndirect).Direct.Clone();
            dstList.Add(obj);
            array.SetAt(i, obj, dstList);
        }
        if (obj.Is<PdfTypeDictionary>())
            CloneIndirectObjects(obj.As<PdfTypeDictionary>(), dstList);
        else if (obj.Is<PdfTypeArray>())
            CloneIndirectObjects(obj.As<PdfTypeArray>(), dstList);
    }
}

// firstly we should clone dictionary (or array) in usual way using API.
var dict = srcTextObj.Font.Dictionary.Clone(false) as PdfTypeDictionary;  // <----------- FALSE should be passed to the Clone method
        
// Next recursively clone all indirect objects inside the cloned one.
var dstList = PdfIndirectList.FromPdfDocument(dstDoc);
CloneIndirectObjects(dict, dstList);
        
//Thats all. Now the dict may be used in other document.