Even if 999 of all text cannot be selected but you have one character of renderable text on that page the OCR routine will not work.
What is renderable text in pdf. Thats the impression youll. Renderable text is the text youd have as PDF page content when you output a PDF from an authoring file say an MicroSoft Word file. Renderable text is editable text that exists within the PDF file.
It may be that whatever you use to scan is adding at least one character of text which may be hidden - eg no stroke -. I have Adobe Acrobat 9 Pro. A definition for renderable text is here.
To enjoy this app though you will have to buy the paid version. The way this text is encoded into the page can cause Acrobat to disallow additional searchable text OCR text. Its a collection of glyphs.
Renderable text is true text in a PDF file - or more specifically in this case on a PDF page. Some of the documents that cause the renderable text error look as if they were generated by a computer born digital as some are saying these days but either some of the text is not selectable or it is selectable but the copied text is gibberish. HOW TO EDIT RENDERABLE TEXT IN A PDFs interface is simple to operate and comes with animated characters that guide you through the program.
OCR can only function on a PDF page that has only an image of text. I cannot OCR those because its not an image. You can test this by selecting text.
Then you run OCR Optical Character Recognition over the document and the result is text which can be searched. It is as if you are modifying the OCR PDF as if it is really a text file. In this way you will be able to edit or modify the said texts as you want even with out MS Office.