Double-click on a PDF document to open it. Your computer will open Adobe's Reader program, which will then open the file. Alternatively, open the Reader program and open a PDF document from the "File" tab.
Click the "Rotate Left" or "Rotate Right" button in the toolbar of Adobe Reader. It doesn't matter which one you select. This will rotate the Adobe document 90 degrees in the selected direction.
Click the same button one more time to rotate the document another 90 degrees, resulting in an upside-down document.