Recursively Changing File extensions from .docx and .pdf to .txt - powershell

Question

$findPDF = Get-ChildItem -Path "$fileDrive" -Filter *.pdf -r
$findDOCX = Get-ChildItem -Path "$fileDrive" -Filter *.docx -r
$pullFiles += $findPDF
$pullFiles += $findDOCX
#[array]$pullFiles
#$pullFiles.length
$holdPath = #()
for($i = 0; $i -lt $pullFiles.length; $i++){
#get the full path of each document
$fullPath = Resolve-Path $pullFiles.fullname[$i]
#stores the information in a global array
$holdPath += $fullPath.path
}
#$holdPath
<#
.DESCRIPTION Uses the word.APPLICATION object to open and convert the word documents into .txt.
#>
#https://stackoverflow.com/questions/13402898/how-can-i-use-powershell-to-save-as-a-different-file-extension
#wdFormatDOSTextLineBreaks 5 Microsoft DOS text with line breaks preserved.
foreach($fi in $holdPath){
$Doc = $word.Documents.Open($fi.name)
$NameDOCX = ($Doc.name).replace("docx","txt")
$Doc.saveas([ref] $NameDOCX, [ref] 5)
$NamePDF = ($Doc.name).replace("pdf","txt")
$Doc.saveas([ref] $NamePDF, [ref] 5)
$Doc.close()
}
The problem Statement
The program needs to get any pdf and doc/x file and convert it to a .txt file. Now, I am able to recursively search and pull all .docx and .pdf documents from the file system. Now, I just need to convert them.
The Error
You cannot call a method on a null-valued expression.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:38 char:2
+ $Doc = $word.Documents.Open($fi.name)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
You cannot call a method on a null-valued expression.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:40 char:2
+ $NameDOCX = ($Doc.name).replace("docx","txt")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
[ref] cannot be applied to a variable that does not exist.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:41 char:2
+ $Doc.saveas([ref] $NameDOCX, [ref] 5)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (NameDOCX:VariablePath) [], RuntimeException
+ FullyQualifiedErrorId : NonExistingVariableReference
You cannot call a method on a null-valued expression.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:43 char:2
+ $NamePDF = ($Doc.name).replace("pdf","txt")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
[ref] cannot be applied to a variable that does not exist.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:44 char:2
+ $Doc.saveas([ref] $NamePDF, [ref] 5)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (NamePDF:VariablePath) [], RuntimeException
+ FullyQualifiedErrorId : NonExistingVariableReference
You cannot call a method on a null-valued expression.
At C:\Users\p617824\Documents\files\powershell\fileExtRename.ps1:46 char:2
+ $Doc.close()
+ ~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull

Answer 1

$word variable are not initialized, and your are complicated for Nothing (without offense you). Modify all your script like this :
$fileDrive ="C:\temp"
$word = new-object -ComObject Word.Application
Get-ChildItem -Path $fileDrive -file -r -Include "*.docx", "*.pdf" | %{
$Doc = $word.Documents.Open($_.FullName)
$NameDOC = $_.FullName.replace(".docx",".txt").replace(".pdf",".txt")
$Doc.saveas([ref] $NameDOC, [ref] 5)
$Doc.close()
}
$word.Quit()
But i have doubt on convert pdf to .txt with Word application like this... I think you should use itextsharp libray like here