Open the PDF.
Tap on the thumbnail button (which is on the top left).
The view now changes so the pages of the PDF are arranged as thumbnails and the thumbnail button will be highlighted.
Tap on the edit button which is in the bottom-left corner of the screen.
Select the page(s) that you want to rotate.
Tap on the rotate button (the 3rd button). This will rotate your page by 90° to the right.