You can copy pages in a PDF document in Preview and paste them to get a new document with only the pages you want. You can split a PDF into two or more smaller documents. You can also merge documents by dragging pages from one document into another.
You can use Preview to easily merge two PDF files, or bring pages from one into the other. You can also create new documents from pages selected from another PDF. You can even bring images into PDFs as new pages.
Learn how to use Preview in Mountain Lion to merge two or more PDF documents into one file. You can also re-arrange the pages in a PDF document or delete pages.