Using Preview, you can merge separate PDF pages. Open the first page in Preview, then select View|Sidebar|View Sidebar. Now open the PDF you want as the next page and make sure the Sidebar is visible for it. Drag the thumbnail from the second page's Sidebar onto the first page's Sidebar and drop it after the first page. Continue for all desired pages. When done, in the original, first PDF page window select either "Save" to save the whole collection as the original page, or Save As to save the collection to a new PDF file.