What does PDFOutline really represent

I have implemented the following recursive method for reaching the outlines:
- (void)loadAllSubChapters:(PDFOutline*)outline {
if (outline == nil) {
return;
}

if (![self.contentLabels containsObject:outline.label]) {
[self.contentLabels addObject:outline.label];
}

PDFOutline *parent = outline;
PDFOutline *child = outline.child;
PDFOutline *sibling = outline.next;

[self loadAllSubChapters:child];
[self loadAllSubChapters:sibling];
}

I have a big pdf file that contains a lot of chapters and subchapters, going two/three levels into depth... To make it more clear here is an example of a main chapter (i.e. it has no parent chapter) containing some subchapters:

<chapter - level 0>

• <subchapter A - level 1>
  • <subchapter A1 - level 2>
  • </subchapter A1 - level 2>
• </subchapter A - level 1>
• <subchapter B - level 1>
  • <subchapter B1 - level 2>
    • <subchapter B1.1 - level 3>
    • </subchapter B1.2 - level3>
  • </subchapter B1 - level 2>
• </subchapter B - level 1>

</chapter - level0>

When I call the loadAllSubChapters: starting with rootOutline (called over PDFDoc's object) I get only chapter titles from levels 0 and 1, which means that all children from chapters from level 1 are nil...

I am really interested why is that so? Could you please tell me what does PDFOutline object really represent?

We've application containing books with incredible dimension (10.000 pages and more) and with huge nested TOC and are handling them without issue.
May you send us one of your pdf file?
May be opening a support ticket?

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