Even if you have text in a PDF which can be selected with a cursor it may not actually be searchable. If you search for a term which you are sure should appear in a document, and nothing comes up, verify that the text you can select and copy to your clipboard is actually searchable ASCII text. Acrobat, or another PDF Viewer may not be able to convert the text embedded in the PDF.
If this happens, try these solutions:
Under Edit . . . Preferences . . . Search, click the 'Purge Cache Contents' option.
2. Under Tools . . . Protection, run 'Sanitize Document'. This utility may remove hidden text.
If neither of these steps works, you may need to write the PDF as a new image and re-OCR. The problem can stem from a corrupted Unicode map. Acrobat uses two tables to store fonts - one for the shapes on the image, and another for the Unicode text. Software can interfere with the font mapping intentionally and render the PDF unsearchable.