1. Open a PDF file, and choose Organize/Home > Insert > From Open Files, or right-click one page thumbnail and choose Insert Pages… > From Open Files.
2. (Optional) directly drag the page thumbnails from an open PDF file and drop at the target place of the current PDF file.
3. In the pop-up Open PDF Files dialog box, select files you want to insert and click OK to continue. (See also “Insert Pages from File”.)