Text must be have OCR (Optical Character Recognition) applied,or it is simply an image.
If apply OCR then the text become editable and you can select, copy, and search text.
I do not know what version of Acrobat you have,but in my case,Acrobat XI, it's under Tools > Text Recognition.
Hi, how can I edit or work with text (highlight, copy...) in document which was created as PDF by scaning from the book?
Perhaps, you could load the scanned document in photoshop, photoshop elements or even in Corel's Paintshop Pro. All of these programs can be downloaded for trial use for 30 days and you never know you could be tempted to buy them because they are so good that you would want them to do wonderful things with them.