Images in scanned documents
Often, pdfs of scanned books will have a number of visual artifacts left by the scanning process. These will frequently be tagged as images by the Make Accessible utility. To tell Acrobat to ignore these images, select the image in the Order Panel, right click, and choose Tag as Background / Artifact.
Cropping scanned documents
Another issue common to scanned documents is that the page will be surrounded by a thick border where there was no scan information. Additionally, the book may have been scanned such that each page of the pdf is two pages of the book. These issues can both be solved by cropping the pages in the Edit PDF tool. For scans where the pages need to be separated, open the Organize Pages tool, and make a duplicate of each page. Then, you can go into the Edit PDF tool, and crop each page to remove the border as well as the extra page.