VBA Macro gets slower with every execution - performance

Question

I have a macro that reads a .txt file line by line. I check every line if it's equal to some code for a new page, say it's "NEXT" - if yes, then a page break is inserted. After certain amount of "NEXT" occurences the whole document gets exported to pdf. Then the content of .doc gets deleted and I continue reading & exporting the txt file until EOF.
Problem: macro gets slower with every execution.
My test file has 27300 lines / 791 kB (real files are somewhere between 10 and 100MB). Before I start the macro, the WINWORD process takes 40K of memory. The memory usage gets bigger after every execution of the macro.
Time Max.MemoryUsage MemoryUsageAfterwards
Run1 11.9s 70K 64K
Run2 19.7s 90K 84K
Run3 22.3s 99K 92K
I know a partial solution is to close and reopen the .doc file manually and run the macro with next .txt file as input. However, it takes long time to even close the Word after one run of macro, although there is no content in the file that I can see.
What I'm asking is if there is another way to solve this, what I believe is to be a memory clearing issue?
My code:
When the document is opened:
Private Sub Document_Open()
ReadAndSplit
End Sub
Global variables and declarations:
Option Explicit
'---------------------------------------------------------------------------
' GLOBAL VARIABLES
'---------------------------------------------------------------------------
Public numOfBreaks As Integer ' number of page breaks made
Public numOfPdfs As Integer ' number of currently printed pdf
Public filePrefix As String ' name prefix for .pdf files
Public sFileName As String ' name of Input File
Public breakAfter As Integer ' print after this number of NEXT
Public cancelActive As Boolean ' cancel Button pressed? (for exit)
Main macro:
Sub ReadAndSplit()
'---------------------------------------------------------------------------
' VARIABLES
'---------------------------------------------------------------------------
Dim sLine As String ' line from text file
Dim numOfLines As Long ' number of lines read from .txt input
Dim execStart As Single ' starting time of script execution
Dim nextPage As Boolean ' indicates if new document beginns
'---------------------------------------------------------------------------
' INITIAL PROCESSING
'---------------------------------------------------------------------------
Application.Visible = False
Application.ScreenUpdating = False
Selection.WholeStory ' clear the document
Selection.Delete
UserForm1.Show ' show user dialog
If cancelActive Then ' Cancel button handling
Application.Visible = True
Exit Sub
End If
With ActiveDocument.PageSetup ' set page margins & orientation
.TopMargin = 0.1
.BottomMargin = 0.1
.LeftMargin = 0.1
.RightMargin = 0.1
End With
'---------------------------------------------------------------------------
' MAIN PROCESSING
'---------------------------------------------------------------------------
numOfBreaks = 0 ' GLOBALS
numOfPdfs = 1
numOfLines = 0 ' LOCALS
nextPage = True
execStart = Timer
Open sFileName For Input As #1
Do While Not EOF(1)
If nextPage Then ' write 2 empty lines
Selection.TypeText (vbNewLine & vbNewLine)
nextPage = False
End If
Line Input #1, sLine ' read 1 line from input
numOfLines = numOfLines + 1 ' count lines
If sLine <> "NEXT" Then ' test for NEXT
Selection.TypeText (sLine) & vbNewLine ' write line from input .txt
Else
Selection.InsertBreak Type:=wdPageBreak ' NEXT -> new page
numOfBreaks = numOfBreaks + 1 ' count new receipts
If numOfBreaks = breakAfter Then ' compare with PARAM
PrintAsPDF ' export to pdf
numOfBreaks = 0
End If
nextPage = True ' switch new page on
End If
Loop
If numOfBreaks <> 0 Then ' print out the last part
PrintAsPDF
End If
Close #1
Debug.Print vbNewLine & "-----EXECUTION-----"
Debug.Print Now
Debug.Print "Lines: " & numOfLines
Debug.Print "Time: " & (Timer - execStart)
Debug.Print "-------------------" & vbNewLine
Selection.WholeStory ' clear the word document
Selection.Delete
Application.Visible = True
End Sub
Macro used for printing PDF:
Sub PrintAsPDF()
Dim newPdfFileName As String ' path + name for current .pdf file
newPdfFileName = ActiveDocument.Path & "\" & filePrefix & "-" & numOfPdfs & ".pdf"
Selection.WholeStory ' set font
With Selection.Font
.Name = "Courier New"
.Size = 10.5
End With
ActiveDocument.SaveAs2 newPdfFileName, 17
numOfPdfs = numOfPdfs + 1
Selection.WholeStory
Selection.Delete
End Sub
UserForm code:
'---------------------------------------------------------------------------
' OK BUTTON
'---------------------------------------------------------------------------
Private Sub OKButton_Click()
Dim inputFileOk As Boolean ' input file path
Dim inputSplitOk As Boolean ' split
Dim prefixOk As Boolean ' prefix
If FileTxtBox.Text = vbNullString Then ' validate file path
inputFileOk = False
MsgBox ("File path missing!")
Else
inputFileOk = True
End If
If IsNumeric(SplitTxtBox.Text) Then ' validate breakAfter
breakAfter = SplitTxtBox.Text
inputSplitOk = True
Else
MsgBox ("Non-numeric value for SPLIT!")
End If
If PrefixTxtBox <> vbNullString Then ' validate prefix
filePrefix = PrefixTxtBox.Text
prefixOk = True
Else
MsgBox ("Missing prefix!")
End If
' check if all inputs are ok
If inputFileOk And inputSplitOk And prefixOk Then
cancelActive = False
Unload Me
End If
End Sub
'---------------------------------------------------------------------------
' CANCEL BUTTON
'---------------------------------------------------------------------------
Private Sub CancelButton_Click()
cancelActive = True ' for script termination
Unload Me
End Sub
'---------------------------------------------------------------------------
' FILE BUTTON
'---------------------------------------------------------------------------
Private Sub FileButton_Click()
Dim i As Integer ' file selection index
' show file chooser dialog and assign selected file to sFileName
Application.FileDialog(msoFileDialogOpen).AllowMultiSelect = False
i = Application.FileDialog(msoFileDialogOpen).Show
If i <> 0 Then
sFileName = Application.FileDialog(msoFileDialogOpen).SelectedItems(1)
FileTxtBox.Text = sFileName
End If
End Sub

Answer 1

Word stores a lot of information in "temp" files in order to track "unlimited" Undo. If you perform a lot of actions without saving the file or clearing the Undo buffer, this slows Word down. I recommend, therefore:
Clear the Undo buffer (ActiveDocument.UndoClear)
Save the (empty) document periodically
in order to free resources.