The most common cause of this issue is you may have the basic driver installed (which does not support PDF scanning), rather than the full feature software and driver installed. To install the full feature software and driver, go to the hp website:
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?product=3441026&lc=en&cc=us&dlc=en&cc=us&lang=en
Then, select your operating system (windows xp, windows 7, windows 8, etc), click next, scroll down until you see "drivers", then select "download."
I hope this helps!