OK, I found it myself:
1. Tools - Pages - Edit Page Design - Header & Footer - Add Header & Footer.
2. Select the font and size, etc, place the cursor on the appropriate site to insert the page number, click the "Insert Page Number" button, and click OK.
Yes, you can effortlessly add page numbers in PDF document and insert numbers in PDF files with the help of PDF Bates Numbering tool. This application is more beneficial for forensic experts and professionals.
how do you do this in the new Adobe Acrobat Cc 2015?