Is there a way to enable viewport margin so it's easier to know I'm at the top of the document?
As stated in the subject, adding a slight margin in the viewport of Acrobat Reader enhances the usability of the UI by allowing the user to know immediately that they've reached the top of the document.
As demonstrated in the comparative screenshot below between MS Word and Acrobat Reader, one has a better user experience (UX) than the other by visually indicating that the top of the document has been reached using a simple margin in the viewport.

