While there are many commercial tools available which allow you to modify/rotate a PDF document and save the changes, you have to download and install the software -- too much of a hassle for a one-off kinda task such as this. Luckily, the fine folks at http://www.rotatepdf.net/ allow you to have your PDF file rotated online to a desired degree (90 clockwise/counterclockwise and 180 clockwise) online, for free.
I just had this issue today and found the website. Once you specify the PDF you want to rotate and the degree of rotation, you click the "Rotate PDF" button. That uploads your PDF to thier servers and performs the requested rotation. Momentarily, you're greeted with a result page allowing you to view the result online or download the rotated PDF to your computer.
While they say they remove the PDF after conversion and don't retain it on their servers, I'd not upload a sensitive document for conversion.