Out of memory exception when merging multiple pdf - vb.net

Question

I have to merge multiple pdf's into one pdf. I'm using iTextSHarp 5.5.10.0 to accomplish this, and it work fine when I merge small files( 2 or more, with 20 pages each), and even work when I try to merge 2 files, one with 3000 pages and the second with 1300 pages, but when I try to merge 3 pdf's with 3000 pages each, I get an out of memory exception.
I don't know how to solve this.
I'm using fileStream and not memoryStream.
I took the code from the answer to this question:
VB.Net Merge multiple pdfs into one and export
Public Function MergePdfFiles(ByVal pdfFiles() As String, ByVal outputPath As String) As Boolean
Dim result As Boolean = False
Dim pdfCount As Integer = 0 'total input pdf file count
Dim f As Integer = 0 'pointer to current input pdf file
Dim fileName As String
Dim reader As iTextSharp.text.pdf.PdfReader = Nothing
Dim pageCount As Integer = 0
Dim pdfDoc As iTextSharp.text.Document = Nothing 'the output pdf document
Dim writer As PdfWriter = Nothing
Dim cb As PdfContentByte = Nothing
Dim page As PdfImportedPage = Nothing
Dim rotation As Integer = 0
Try
pdfCount = pdfFiles.Length
If pdfCount > 1 Then
'Open the 1st item in the array PDFFiles
fileName = pdfFiles(f)
reader = New iTextSharp.text.pdf.PdfReader(fileName)
'Get page count
pageCount = reader.NumberOfPages
pdfDoc = New iTextSharp.text.Document(reader.GetPageSizeWithRotation(1), 18, 18, 18, 18)
writer = PdfWriter.GetInstance(pdfDoc, New FileStream(outputPath, FileMode.OpenOrCreate))
With pdfDoc
.Open()
End With
'Instantiate a PdfContentByte object
cb = writer.DirectContent
'Now loop thru the input pdfs
While f < pdfCount
'Declare a page counter variable
Dim i As Integer = 0
'Loop thru the current input pdf's pages starting at page 1
While i < pageCount
i += 1
'Get the input page size
pdfDoc.SetPageSize(reader.GetPageSizeWithRotation(i))
'Create a new page on the output document
pdfDoc.NewPage()
'If it is the 1st page, we add bookmarks to the page
'Now we get the imported page
page = writer.GetImportedPage(reader, i)
'Read the imported page's rotation
rotation = reader.GetPageRotation(i)
'Then add the imported page to the PdfContentByte object as a template based on the page's rotation
If rotation = 90 Then
cb.AddTemplate(page, 0, -1.0F, 1.0F, 0, 0, reader.GetPageSizeWithRotation(i).Height)
ElseIf rotation = 270 Then
cb.AddTemplate(page, 0, 1.0F, -1.0F, 0, reader.GetPageSizeWithRotation(i).Width + 60, -30)
Else
cb.AddTemplate(page, 1.0F, 0, 0, 1.0F, 0, 0)
End If
End While
'Increment f and read the next input pdf file
f += 1
If f < pdfCount Then
fileName = pdfFiles(f)
reader = New iTextSharp.text.pdf.PdfReader(fileName)
pageCount = reader.NumberOfPages
End If
End While
'When all done, we close the document so that the pdfwriter object can write it to the output file
pdfDoc.Close()
result = True
End If
Catch ex As Exception
Return False
End Try
Return result
End Function
I get the Exception from this line, in the end of the while, when trying to read the second file:
reader = New iTextSharp.text.pdf.PdfReader(fileName)
How can I solve this?