The program supports inserting pages from another PDF file or inserting blank pages into your current PDF file.
Open one file and click the "Organize Page" button on the top left.
In the pop-up window, click the "Insert" button.
Select "Insert Blank Page" on the right-side panel, and set the location of the new blank pages. Click "Insert" to apply such action.
Select "Insert from File" on the right-side panel, and browse to select another PDF file that you want to insert. Please also set the location where the new pages will be added, then click "Insert". This action enables you to insert all pages of the inserted files to your current PDF. You can delete the pages you do not want later.