Note: To bypass the stationery picker when inserting multiple blank pages with the same stationery, choose Edit > Insert Blank Page Again or copy and paste pages. (Creating Blank PDFs).
Drop the new page anywhere on the page currently on view, and it will insert before that current page.
Drop your new page in a specific location in the PDF by using the Thumbnail view in the Sidebar. Hover between two thumbnails until you see a line appear either on the right or left of a thumbnail, then drop the page in.
Note: If you drop on the thumbnail's left, the new page will insert before the page in the thumbnail. If you drop on the thumbnail's right, the new page will insert after the page in the thumbnail.
You can select multiple pages to add:
Once selected, drag the pages from the Sidebar of one PDF to another, making sure that you drop them between, before, or after the pages where you want them inserted.