The controls are easy to use!
From left to right:

+ or - makes the image larger or smaller.
Click the grid symbol to show page navigation on the left of the screen (recommended).
Click the printer icon to print the document; click the arrow pointing down to download it.
Click the broken square to see the document full-screen (when viewing in full-screen mode, the symbol will turn to a broken cross symbol – click to return to standard view).
Click the magnifying glass to search the document; click the curvy arrow to forward it to someone else.