With the Option key held down, only the current page will rotate. (If you save the document, it will be saved with the single page rotated.) ...

Then click the "Rotate PDF" button to complete the rotation and save the PDF in the usual way to complete the process.

How to Rotate the Text on a PDF File. If a client emailed you a PDF with a combination of pages in portrait and landscape mode, some pages may display at a 90-degree angle.