Mega PDFer med liknande namn till 1 fil

Download pdftk (direct link here).
Extract it somewhere.
Put pdftk.exe and the batch together.
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"
)

@echo off
setlocal enabledelayedexpansion
FOR %%A IN (%*) DO (set command=!command! %%A)
pdftk.exe %command% cat output "%~dp1binder.pdf"

@echo off
setlocal enabledelayedexpansion

rem source root folder = script folder
set source=%~dp0

rem destination folder
set "destination=%source%merged"

echo !destination!

for /f "delims=" %%a in ('dir /b /a-d /o:n "%source%"\*.pdf') do (
set x=%%a
set "prefix=!x:~0,6!"


if not "!last_prefix!" == "!prefix!" (
set _pdffiles=
)

set _pdffiles=!_pdffiles! "%%a"

if not "!last_prefix!" == "!prefix!" (
if not defined !last_prefix! (
pdftk.exe !_pdffiles! cat output  "%destination%\!prefix:~0,6!.pdf"
)
)

set "last_prefix=!prefix!"
)