A full screen option is currently not available. You can either open pdf's in your pdf viewer by clicking on it or hide the two Sidebars with F10 and F11 to make more space for the integrated pdf viewer (only works well if the Editor maximum width under Tools > Options > Appearance is set to 0).
Do you zoom using Crtl + & Ctrl -? The pdf viewer has it's own + and - to zoom, then it should definitely change the character size in the pdf itself...
Edit: Your version is kind of outdated, please update to 2.8.8