First, before starting the game, open your digital strategy guide and maximize the PDF. Leave it open when you start the game.
While you are playing the game, if you want to reference the strategy guide, press one of the “Windows” buttons on your keyboard (located on either side of the space bar, it has a little Windows logo and usually the word “Start” on it). This will suspend the game and you will not lose your place. If you left the guide maximized, it should show up immediately, and the game window will be minimized.
(You can also use the key combination ALT + TAB to minimize the game and return to your desktop if you are more familiar with that option, instead of the “Windows” button.)
If the guide is not visible, click on the strategy guide PDF button on the task bar at the bottom of the screen to maximize it.
When you are ready to return to the game, just click on the game button on the bottom task bar. This will take you back to the game right where you left off.