Buying a scanner costs more but scanning a document using your phone is free. Now select your option !! The Android application, which can be used to scan and create PDF is CamScanner. Using this app you can take photo using different modes like document scanning and image scanning. I have scanned few of my childhood photos, it's awesome.. Give a try. The picture gives explanation more than what i can.
If possible i will upload the video explaining ,how to do that .
The link to download this app : https://market.android.com/details?id=com.intsig.camscanner&hl=en