To edit a document, you have to import it by selecting it from your hard drive. Once you've selected it, you'll see thumbnails of all the document's pages.
By selecting the pages you want to edit, you can rotate them, change their order, trim, or delete any of the pages without affecting the other pages.
When you're finished, you can export the modified document as a new PDF file.
If you modify PDF files often, PDF-Shuffler is an incredibly useful tool to have.