Your Yubikey comes pre-configured for releasing a One Time Password (OTP) i.e. a new unique password each time the Yubikey button is pressed. This mode is mainly designed to be used access control where a server calculates and verifies the OTP used. However, a Yubikey can in addition to the OTP mode be configured to (using the second configuration slot, a programmable feature of your Yubikey) provide a so called Static Password that can be used for applications requiring static passwords such as Microsoft Office. You can configure a long static Password (or have the Yubikey Personalization tool create one for you). Then each time you want to protect a document you will press the button for about 4 seconds (releasing the static password from the second configuration slot of the Yubikey) to protect the document and pressing the button again when you want to unlock a document.
To program the second configuration slot of the YubiKey, please download the latest YubiKey Personalization Tool from the link below:
http://www.yubico.com/personalization-toolEach person you would want to share the document with would naturally also need the same password, either entered manually or configured in their Yubikey so this approach will in practice mostly work for sharing password protected documents within a smaller group of people.