Although the official installation guide recommends you to install Pdftk by copying the 2 required files** to c:\windows\system32, I personally prefer to keep application specific files outside Windows’ core directory.
Thus, in order to do so simply create a program directory such as C:\Program Files (x86)\pdftk and extract all files from the downloaded zip into it, resulting in the following folder structure:
C:\Program Files (x86)\pdftk -> .\bin\ -> .\docs -> .\license_gpl_pdftk
Now you can simple add the bin-folder to the system PATH and you are ready to go. Using this approach it’s easier to update the core Pdftk files without the need to access any Windows’ core folders.
** libiconf2.dll and pdftk.exe