Merge PDF files with related filenames automatically

Question

I need to merge/combine several PDF files.

What I have is multiple PDF files in one folder. The PDFs that need to be clubbed/combined together would have a partial file name in common; for example:

123456_ABCD.pdf
123456_EFGH.pdf
123456_IJKL.pdf 

The file name would start with numbers then underscore then file name. I'm going to have hundreds of similar PDFs, and I'm looking for a way to automatically merge these files using a batch script, VBA in Excel, or whatever.

The output of the new file should be in a separate folder with the name 123456.pdf. I use a Windows OS. I have Adobe Reader, PDF995 and CutePDFwriter installed in my system.

I can manage to get all the pdf's in one main folder. That folder would have multiple pdf's like below.

123456_ABCD.pdf
123456_EFGH.pdf
123456_IJKL.pdf
111111_ABCD.pdf
111111_EFGH.pdf
222222_IJKL.pdf
222222_WXYZ.pdf

In this scenario the pdf's that I am expecting in the output folder are:

123456.pdf
111111.pdf
222222.pdf

Answer 1

(Untested)

1. Download pdftk (direct link here).
2. Extract it somewhere.
3. Put pdftk.exe and the batch together.
4. Then open cmd where the batch is located* and run it.
   _{* (shift+RightClic open command prompt here)}

The script should works if the folder structure is like this:

|
+---111111
|       111111_ABCD.pdf
|       111111_EFGH.pdf
|
+---123456
|       123456_ABCD.pdf
|       123456_EFGH.pdf
|       123456_IJKL.pdf
|
\---222222
        222222_IJKL.pdf
        222222_WXYZ.pdf

And not in a single folder where all the files are there.

@echo off
setlocal enabledelayedexpansion

rem source root folder where to crawl pdfs.
set "source=c:\mydoc\pdf"

rem destination folder
set "destination=c:\mydoc\merged"

for /f "delims=" %%a in ('dir /b /s /ad /o:n "%source%"') do (
    set _pdffiles=
    for /f "delims=" %%i in ('dir /b /a-d /o:n "%%a\*.pdf"') do (
        set _pdffiles=!_pdffiles! "%%i"
        set "_outputpdf=%%~ni"
    )
    echo pdftk.exe !_pdffiles! cat output "%destination%\!_outputpdf:~0,6!.pdf"
)

Remove echo if you think the batch is Okay.

Answer largely inspired by this

Update: There are two answers that complement my post