Get size of another application to place window scaled properly above it

Background:

I work a lot with pdf's that are heavy in graphics. In fact, they are graphical representations of code. Sometimes I want to compare versions of the code, usually it's done by opening two pdf readers and swap between windows or put them next to each other.

I got tired of that so I whipped up a simple app to contain a clipped image of the page of one pdf and made it semi transparent and put it above the other pdf. It works for me on my machine but someone else will have another resolution and placement of his reader so it won't work as well.

Question: Can I get the position and size of another app (e.g. Acrobat Reader) to put mine on top of it scaled properly?

Or is there a better way to do this? I can see two alternative, either I could write a plugin f开发者_运维百科or Acrobat Reader or some other pdf-reader, or I could make my own reader and make it able to open two pdf's on top of each other with one semi transparent.