While most scanned documents can be OCR’d, the best scan settings for documents for optimal OCR results should be in grayscale and have a resolution of around 300 dpi with font sizes of 10 points or larger, or 400-600 dpi with font sizes of 9 points or smaller.
To improve your scan for better OCR results, see below:
Is your document particularly complex? For example, are there multiple different fonts and types of formatting? How is the quality of your document? Typically, the quality of the original document affects the quality of the OCR performance. Crisp, clean originals with clear text will produce much better results than crumpled, faded photocopies. Before performing OCR, choose Edit > Deskew and Adjust Image to straighten the image and to adjust image contrast and exposure. Better contrast between the type and the background will result in better OCR results. You can find more information on scanning in our Help manual.
Otherwise, the issue may be that you are working with vector graphics (images embedded onto the document like a watermark). In these cases, text cannot be selected. To edit your document, use the Text tool and create a text imprint box with solid background fill and type over the existing text. If you would like us to take a look at the document and confirm the issue, please send us a sample and we'd be happy to look further into the issue.