OCR text recognition error

I am having a problem figuring out the easiest way to search and highlight a pdf file on Adobe Acrobat XI Standard. I am trying to get through hundreds of pages of medical records and I tried doing an OCR but at least half of the pages are coming up as an error, specifically saying "unknown error." The only way I can think to fix it would be to rescan the pages that have errors, but it would take soooo much time. Is there a way to fix this error? Or is there a way to search the pdf file as a scanned doc? Please help!