GetAnnot(nIndex) returns the same annotation though variable nIndex is incremented

In "test.pdf", there are 30 annotations, but "GetAnnot(nIndex)"  can just returns the first 2 annotations, though variable "nIndex" is incremented from 0 to 30,  variable "N" repeat 0 and 1.

The pdf file is fine, and other pdf files have the same problem, What's the problem?

Acrobat X , VB

Code:

Private Sub Form_Load()
Dim PDDoc As Acrobat.AcroPDDoc
Dim PDPage As Acrobat.AcroPDPage
Dim PDAnnot As Acrobat.AcroPDAnnot
Dim NumAnnots As Long
Dim NumPage As Long
Dim I As Long
Dim nIndex As Long

Dim AnTxt As String
Dim N As Long

Set PDDoc = CreateObject("AcroExch.PDDoc")
PDDoc.Open "D:\test.pdf"
NumPage = PDDoc.GetNumPages
If NumPage > 0 Then
    For I = 0 To NumPage - 1
       Set PDPage = PDDoc.AcquirePage(I)
       NumAnnots = PDPage.GetNumAnnots()
       If NumAnnots > 0 Then
          For nIndex= 0 To NumAnnots - 1

              Set PDAnnot = PDPage.GetAnnot(nIndex)
              N = PDPage.GetAnnotIndex(PDAnnot)

              AnTxt = PDAnnot.GetContents()
          Next
       End If
    Next
End If
PDDoc.Close
End Sub