There is a choice of 2 variants of the same product (you only need one of them):
The download is fairly large (100s of MB), but once installed the software can be used without an internet connection and does not require registration. Compatibility with the MS Office formats is very good, but you may find problems with particularly complex documents.
These are much smaller downloads, but have the downside that they do not allow you to make changes to documents. No registration required and once installed on your PC, you can view documents quickly without an internet connection.
There are several on-line web browser based tools for viewing and editing MS Office format files. Most require registration (free) and you usually need to be connected to the internet when using them. Functionality may be more limited than with an office suite installed on your computer, but there is nothing to download / install so it's quick and easy.
These are some of the popular ones:
This is another useful website converts between various file formats (e.g PDF to .doc, images):