Hope you are doing well and sorry for the trouble.
Is this an issue with a particular PDF file or with all the PDFs? Please try with a different PDF file and check.
What is the version of the Acrobat DC you are using? To check the version of the application go to Help > About Acrobat and make sure you have the recent version 23.01.2006x installed. Go to Help > Check for updates and reboot the computer once.
As described the PDF has 4600+ pages of text and images, it may take up a minute for the application to process the request and apply stamp on the PDF file.