I scanned a pdf document but don't know where it got saved in my Mac book. Can someone help me pls?

In your Scanner's software "Settings" that you adjust just before you scan an image, there is a box where you set where the Saved Image will be Saved. Open youre Scanner's software and check that setting.

In Finder's sidebar, click on 'All My Files'. It should be listed at the top, assuming you've just scanned it (if it was in the last 7 days, scroll through the list, it should be easy enough to spot).

Also, K, you can use Spotlight to search, and specify you only want it to find PDFs.