Why does iPad show a weird effect on some PDF images?
Though I can't say for certain without examining the actual PDF (or at least that page), I suspect the apparent artifacts -- the lines -- are the result of the image containing an imperfect grid of pixels, probably due to a slight resizing. 

I created an example to demonstrate the idea. It doesn't reproduce exactly what you're seeing on the iPad, but the concept is the same.

Here are two grids of black and white pixels, one perfect and one imperfect. Zoomed at 600% you can clearly see the difference between them:

Perfect Pixel Grid (600% Zoom)



Imperfect Pixel Grid (600% Zoom)



If you look at those same images at their original size they look about the same:

Perfect Pixel Grid (100% Zoom)



Imperfect Pixel Grid (100% Zoom)



However, if you resize them the way the iPad does, effectively putting the pixels at the nearest spot it can find after you change the size, you can see the difference easily -- basically the math doesn't work out perfectly because you can perform zooming of any percentage with the gesture. The effect is called aliasing:

Perfect Pixel Grid with Aliased Zoom



Imperfect Pixel Grid with Aliased Zoom