This is the solution (via ALP)

Acrobat Won’t OCR your file because it contains renderable text You’ll see the Renderable Text Error when the PDF you are trying to OCR has vector elements on it like stamps, annotations or Bates Numbers. It’s a particular problem with federal court files that are image-only PDFs with stamped Bates numbers. Solutions: Remove Headers and Footers or Bates Numbers Go to Document—>Add Headers and Footers and remove the headers and footers and remove all entries.This solution only works if the Headers and Footers or Bates Numbers were stamped using Acrobat 8. Remove the Header/Footers Manually You can select and delete the vector elements by choosing Tools—>Advanced Editing—>Touchup Object Tool in Acrobat 8 Professional. Another option is to use the Redaction Tools in Acrobat 8 Professional to remove them.

Also:

Regarding the renderable text issue: You can workaround this problem by rasterizing the entire PDF, and re-PDF’ing it. In other words, convert it to an image file and then back to a PDF. This eliminates any added headers/footers, page numbers, or other unknown renderable text.

For example, you can do a SAVE AS, and use the TIF file format. This will save each page in your PDF as a separate TIF file. Then in windows explorer, highlight all of the TIF files and select “Combine in Adobe Acrobat” - this will convert them all back to PDF and combine them into one file. Save the new PDF file, and then you will be able to run OCR on it without getting the renderable text error. -