I have a Canon scanner that allows my PDF pages to be highlighted in Reader X, but I also have a Dell multifunction laser printer/fax/scanner that does not allow me to highlight scanned PDF pages. Why is the highlight feature available in the Canon scanner but not the Dell?
When you scan with the Canon software, it can create a PDF that includes text as a result of an OCR operation. It is this text that Reader is able to highlight. The Dell scanning software is likely not performing OCR for the PDFs it creates, so there is no text in the PDFs to highlight, only an image of text.