What's taking so long is that PDFs are generally stored as vector art and your computer is taking a long time to trace all of the curves and save the result as a bitmapped image. The more complicated the artwork (this includes text), the longer it'll take to "rasterize" it all.
Here's a little primer in vector versus bitmap images.
A bitmap of a diagonal line may be stored like this:
0000001
0000010
0000100
0001000
0010000
0100000
1000000
...where the 1s draw a line. The image is "blitted" to the screen, which means they'll just directly copied from memory onto the screen.
A vector of the same diagonal line may be stored like this:
line(0,6,6,0)
...where it's a command and a set of coordinates describing where to put it. When this is displayed on the screen, the computer traces the line over a virtual grid. Once all of the elements have been traced, the result is blitted to the screen. This is why a PDF graphic can be zoomed infinitely and always stay sharp; it's redrawing every piece of the image each time from scratch with a differently scaled grid. This tracing/drawing process into a bitmap is called rasterizing.
As you might be able to guess, rasterizing requires a
lot more compute cycles than simple blitting. That's why displaying PDFs on older machines can be painfully slow.
I hope this helps.
