OCR and PDF

The Adobe PDF file format provides two ways to combine in a single file images and OCR’ed text, or images that have been converted to text through Optical Character Recognition (OCR) software.

(1) The "image with hidden text format" stores the complete original image of a scanned document, along with the text obtained through OCR. The text is "hidden" in the sense that simply opening the PDF file displays only the scanned image, not the underlying OCR'ed text. Because the OCR'ed text is "hidden" in the file, however, dtSearch can index and search it.



After a search, when a user clicks on an "image with hidden text format" PDF document, the dtSearch product will display the scanned image. Because the actual OCR’ed text is "hidden," the display will appear to highlight hits directly on the image. Click here for a dtSearch Web demo showing hidden text highlighting.

(2) Another option for combining scanned images and OCR’ed text in a single PDF file uses "small images" for the parts of each scanned page that do not appear to be text. For example, the format would store a picture or a signature as a small image embedded in the page. The format would store the non-picture portion of the page only as OCR’ed text.

While the "small images" alternative does not preserve the "true" image of the original document, it does produce much more compact files than the "image with hidden text" option. The "small images" PDF file usually stores only a few images for each page, instead of a complete image of the whole document. The text detected through OCR in the "small images" format can also be more readable because the resulting PDF file stores it as text with font information rather than as an image.