Update: Look at the download link. It is not official Microsoft website but a website registered to someone in China. The download may even be virus infected and/or cracked Office. STAY AWAY FROM IT!!
If you look on ebay, there are too many people selling MS product keys. Most of them will work sometime and give you the impression of having a legit product, but that is not the case. Easiest is to contact Microsoft over chat and ask them about the product key that you received. They will tell you that it is part of some license deal (e.g. educational license with 500 max installation). Once the maximum number of installation is reached, Microsoft will block the key and you will not be able to reinstall your office again.
According to EU law, you are responsible to ensure that you are buying a legitimate product. If the software is used, then you need from all former owner a statement that they removed any old installation and are not keeping copies of the software. If the product is licensed for many users, like Office Professional Plus, you need copy of original license agreement and description of which part of the license agreement you are buying. E.g. if a company has a 10-user license and sells you the 10th user license, since they only need 9 user license, that is legal and fine. But it has to be made clear to you, what you are buying (e.g. the 10th user of a legitimate 10-user license with proof of purchase).
If you are not in the EU other laws will apply to you.
So you can use it, it will work for some time, even updates will work but most likely after some time you will not be able to reinstall your office, hence it is most likely a stolen license key.