Adjust PDF Contrast with Adobe Reader/Acrobat?

We get PDF's from our professor to read for homework but they're often scanned documents, is there a way to adjust the contrast of the text to make it easier to read?

Edit: I've got Photoshop but is there a way to do it from a PDF reader?

Edit2: Windows XP, 7 ** Windows or Ubuntu Only **

You can try this:

Go to Edit>Preferences>Accessibility

This will not change the true contrast, but you can pick contrasting colors of your choice, or one of the defaults, as in the screenshot.