In addition to importing PDF document into an open document using
options under the
File menu, you can drag PDF documents from
the Finder to the page list and their pages will be inserted into the
open document at the place of the drop.
You can also drag a PDF document directly onto a page in the preview area. This will insert the first page of the dragged document onto the page as a static annotation object, allowing you to resize and otherwise control the appearance and placement of the inserted page.