The default view in the application is Page View, where a Single Page is visible at a time. The other option is Facing Pages view where two pages are visible at once, like a book. Change between these views by clicking on the Sidebar item in the toolbar sidebar button or via the View menu.
Choose which view your documents automatically open with by choosing Preferences > General > Default view. With Facing Pages view your document can open with the first two pages visible together, or with the first page alone on the right hand side.
From the View menu there are also options for:
Full Screen mode (Command+Control+F).
Zoom to Fit (Command+Option+0) which fits your entire page in your current window size
Zoom to Width (Command+Option+9) which zooms the width of the page to fit your current window width.
Actual Size (Command+0) which displays your document at 72 pixels per inch. On a retina display, this means the visible size is about the same as on a regular screen, but with a clearer image.