Right now, the only types of images that can be imported from Dropbox, Google Drive, and other sources are images, such as jpg files and png files.
If you have software to convert your PDF to a jpg file, that will allow you to import it as a background. Alternatively, you could take a screenshot of each page of your PDF on your iPad with the following steps:
1 - Pull up a page of your PDF on the iPad
2 - Press both the home button (plastic circle on the front of the iPad) and the sleep/wake button (the one on the top of the iPad) simultaneously. Don't hold, just press briefly.
3 - The screen should flash white - that means an image of everything on the screen has been saved to your Camera Roll
4 - Import images from your Camera Roll to use the pages you screenshot in your deck