Weston, the 64-bit OS contains an emulation layer making it possible to run 32-bit applications. In fact, most common applications are still 32-bit. For example, the official Firefox builds are 32-bit only. But they run on the 64-bit Windows just fine. Therefore, if your OS is 64-bit, then in fact both files would work.
I am using the 64-bit OS myself (and most new PCs are nowadays shipped with 64-bit Windows pre-installed) and I am trying to use mostly 64-bit software. So in the case of your Help file, I would download the 64-bit version. But the 32-bit version would work as well. The only incompatible combination is the 32-bit OS and the 64-bit software. But since your OS is 64-bit, you don't have to worry about it.