I use Xodo for reading and annotating PDFs. Unlike with e-books, screen real estate with PDFs is a highly valuable commodity since PDFs can't be reformatted on the fly.
I also tend to read PDFs with my tablet pen in hand, annotating as I go. So I stay in annotation mode the whole time.
It would be great then to be able to hide the annotation tool bar in Annotation Mode and allow the PDF to fill the whole screen, in exactly the same way as View Mode.