Magnifiable Images in a PDF document?

I'm viewing a 3-page PDF document received from elsewhere, using Acrobat 7.0 Standard on a MacBook running OS 10.4.11. The document contains 4 figures. When I run the pointer (the "hand") over 2 of these images, a short horizontal bar with a short arrow pointing downward from it appears inside the hand. Clicking once significantly magnifies the size of the image, up to full screen or full window width. What happens with further clicks is a bit confusing, but there is no further magnification, and eventually a demagnification back to the initial size.

Nothing like this happens with the other two figures, and nothing like this happens if I view the document in Mac Preview. Only limited information on how this PDF document was created is available, except that maybe PowerPoint was involved at some stage.

Any ideas as to what's going on here?

Thanks for any info.