A distinctive feature of APEX is interactive, 3D graphics in the .pdf version. Nearly all graphs of objects in space can be rotated, shifted, and zoomed in/out so the reader can better understand the object illustrated.
Currently, the only pdf viewers that support these 3D graphics for computers are Adobe Reader & Acrobat. To activate the interactive mode, click on the image. Once activated, one can click/drag to rotate the object and use the scroll wheel on a mouse to zoom in/out. (A great way to investigate an image is to first zoom in on the page of the pdf viewer so the graphic itself takes up much of the screen, then zoom inside the graphic itself.) A CTRL-click/drag pans the object left/right or up/down. By right-clicking on the graph one can access a menu of other options, such as changing the lighting scheme or perspective. One can also revert the graph back to its default view. If you wish to deactivate the interactivity, one can right-click and choose the “Disable Content” option.
The situation is more interesting for tablets and smartphones. The 3D graphics files have been arrayed at https://sites.und.edu/timothy.prescott/apex/prc/. At the bottom of the page are links to Android and iOS apps that can display the interactive files.