In Adobe Acrobat Reader mobile application, you have an option to delete or clear the annotations that you add in the PDF.
While adding annotations in the PDF, you also get undo icon to revert the changes.
Currently, we do not have any information if there is any plan to add the eraser tool in the application.
However, you may raise a request for this over here Feature Request/Bug Report Form.
Let us know if you have any questions.