How to call PDPageRelease API to release pdf?

Here is my code.

Public Function GetPDFLastTwentyText(ByVal pstrPdfFilename As String) As String 
     Dim PDDoc As Object 
     Dim CAcroRect As New Acrobat.AcroRect 
     Dim PDPage As Acrobat.AcroPDPage 
     Dim PDTxtSelect As Acrobat.AcroPDTextSelect 
     Dim CArcoPoint As Acrobat.AcroPoint 
     Dim iNumWords As Integer 
     Dim iMax As Long 
     Dim arPdfLines() As String 
     Dim i As Integer 
     Dim fso As FileSystemObject 
     GetPDFLastTwentyText = "" 
     Set fso = New FileSystemObject 
     If fso.FileExists(pstrPdfFilename) Then 
          Set PDDoc = CreateObject("AcroExch.PDDoc") 
          PDDoc.Open pstrPdfFilename 
          Set PDPage = PDDoc.AcquirePage(PDDoc.GetNumPages() - 1) 
          Set CArcoPoint = PDPage.GetSize() 
          CAcroRect.Top = CArcoPoint.y 
          CAcroRect.Left = 0 
          CAcroRect.Right = CArcoPoint.x 
          CAcroRect.bottom = 0 
          Set PDTxtSelect = PDDoc.CreateTextSelect(PDDoc.GetNumPages() - 1, CAcroRect) 
          ... 
          PDDoc.Close 
     End If  
     
     Set fso = Nothing 
     Set PDTxtSelect = Nothing 
     Set CAcroRect = Nothing 
     Set CArcoPoint = Nothing 
     Set PDPage = Nothing 
     Set PDDoc = Nothing  
End Function 

I have no idea to use this API.

Although I call API directly "PDPageRelease (PDPage)" in my code, it will get error that it can't find this API.