In ArcSite, when you upload a PDF into the system, you don't have the ability to "edit" the underlying image. There is a solution, covered below.
While you can not edit the elements on an existing PDF, there is a way to cover up unwanted areas of a PDF, export the “new” PDF and import again.
If you imported a PDF in error, you can delete the drawing entirely and re-import the correct PDF file. In order to delete your drawing, you will need to do this via your device, not via the user site.