Processing tips for scanned photos are in my sig. They make a huge difference for PDF.
For instance you could apply a dot matrix after removing the printed dots. I know how that sounds but it does look better afterwards. After it's processed it looks like someone payed attention to it, looks more professional (rather than just scanned and botched together). Alternatively you could blur it and apply a pixelize filter if the image was pixelized in its printed form. Makes a HUGE difference. Sometimes they look better than the printed version, especially if you add some contrast and saturation.
Also, you may want some drawings in SVG format (especially black & white). They're much cleaner and they scale really well no matter the resolution or zoom level. Great for diagrams, charts and some colour drawings too. Inkscape can autotrace, but Vector Magic is better (not perfect but better), and you can always clean up in Inkscape.
Regarding DRM, just go with whatever's available. If someone is really determined to copy the material and post it online they will. The iPad has a screenshot feature, and I'm almost positive the Kindle does too. So anyone who's determined could, theoretically, OCR the screenshots and put together the images in their original places.