You can add blank pages to your PDF documents by right clicking the page list and choosing Add Blank Page from the popup menu (which will add a page at the end), or Insert Blank Page (which will insert a blank page before the first selected page in the list.
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.
You can click the Source or Label columns in the page list to sort the document by that column. Since both the source name and labels for each page are freely assignable this allows for flexible sorting options, especially in combination with the automated relabeling and renaming options available in the contextual menu of the page list.