You're probably using Workbench, i would suggest you to post in the Workbench forum.
I believe workbench saves all resources (xdp, pdf, images, fragments, custom components and so on) on a resource DB that allows for colaborive work, instead of a physical file, but to be honest i have no idea here in with tables it is stored and how you can retrieve it manually.
If you're using Designer then the xdp file is in your local machine (the machine from here Desinger was launched)