What app do you open them in on the iPad?
DropBox uses the iPad's built in QuickLook ability to show files. It's not really meant to be an accurate display of the file, only a way of assuring you that you're looking at the right file.
To view the files accurately you'll need to open them in a more capable app. Pages does a fair job of importing simple graphs. I'm not sure how the other Office Suites do, but the more popular ones are Documents to Go, Office HD, QuickOffice, and Polaris Office. QuickOffice might be on it's last legs for future updates. Google bought them recently, and have not announced what their plans are for the app.
Hold in mind, no mobile app does a perfect job of importing and displaying Office files. If you want a perfect presentation, and a static display is ok, you might prefer to save and display them as PDF files.
