PDFs that are just scanned images of book pages are not accessible, since a screen reader cannot read the words on the page. Making a document machine-readable enables:
This requires Adobe Acrobat X Pro, which is available on staff computers maintained by HCC IT
from MSU Web Accessibility site
In-depth overview covering accessible PDFs
Tutorial from Penn State.
Guidelines from the Social Security Administration (129 pages).
A guide for creating accessible PDF files in a number of different Adobe products (86 pages).
This pamphlet from the Internet Archive includes OCR. Notice that you can select the text of the pamphlet, or CTRL+F to search the document.
A scanned document that has not been OCR'd, and is not accessible. Notice that you cannot select text or search the document.