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PDFs are a great archive format for scanned images, but Acrobat does not allow you to edit the text layer of the document (the part that can copy and paste from) leaving you with just the raw OCR. Are there any freeware alternatives that let you edit the text layer?

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What do you mean exactly by the "text stream"? On a scanned document, the text is an image as well, you can't edit it easily. –  Gnoupi Jun 24 '10 at 15:48
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A PDF file has the potential to store two levels of representation, the actual image and a text part, which is what I (perhaps mistakenly) called the "text stream". When a word processing document is converted to a PDF, this part is created at the same time as the image, and is usually quite accurate. When a scanned document is turned into a PDF, the text part is created by OCR processing of the image. There are also PDF files that have no text part at all. This part is what you are accessing when you copy and paste text from a PDF document. –  Emil Jun 24 '10 at 20:27
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You should add this info to the question ;-) –  Ivo Flipse Jun 25 '10 at 10:56
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I believe in keeping the question brief and to the point, and leaving any additional or clarifying information in the comments. I've edited the question to make it as clear as possible without getting to wordy. –  Emil Jun 26 '10 at 17:32
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3 Answers

Free PDF editors are very scarce.

The only free one I know is OpenOffice with Sun PDF Import Extension.

From the techsupportalert article A PDF File Allows Editing in 100% Layout Accuracy:

Sun PDF Import ExtensionOpenOffice with Sun PDF Import Extension produces a hybrid PDF / ODF file. The file created will have a normal .pdf file extension. By itself, it is a PDF file and can be viewed by any PDF viewer such as Adobe Reader, PDF-XChange Viewer or Foxit Reader.

On top of this, it contains a source ODF file, which can be opened with OpenOffice directly from the PDF file for editing without loosing any layouts, bookmarks, hyperlinks or formats.

To create a hybrid PDF file, run OpenOffice with Sun PDF Import Extension installed, select "File", choose "Export as PDF", a PDF Options window like the screen shot will open, then tick "Create hybrid file" and click "Export".

This hybrid PDF file saves you in keeping two separate file formats as it has combined two into one. It is ready for sharing and viewing with a PDF reader, yet it can be opened for perfect editing just the way a normal OpenOffice file can be. Probably it will be a good idea to name the hybrid file ending with "-odf.pdf" to differentiate from the normal PDF file.

Sun PDF Import Extension is compatible with OpenOffice.org (3.0 or later) or StarOffice 9.

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Great, thanks! This looks very promising, even if rather cumbersome. –  Emil Jun 24 '10 at 19:04
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A scanned document converted into a PDF initially does not contain any text. It's composed of pages each covered by a full-page pixel image. This image may or may not contain areas that look the same as shapes of characters, identified by human brains as letters and "text".

Programmatically, it is not text, only pixels.

In order to insert into a PDF derived from scanned images something that is real text, one can only employ an OCR process. This will add an extra layer of content to the PDF pages. That extra layer would contain all identified (or mis-identified) characters behind the pixel shapes as real glyphs from a real font. However, these real-text characters do have a special PDF markup, tagging them to not be rendered visually by a viewer (or when printing). Their existens would show up only when searching (or highlighting) text (or when trying to copy'n'paste areas from the image while the Acrobat Text Touchup Tool is active).

So, is your real question this: "The OCR results for my scanned PDF documents are sub-optimal. Not all characters are correctly identified. I want to edit the hidden text in order to make OCR result better. How do I do that with a free tool?" ?


Edit: I'm not normally using Acrobat. But just now I had the opportunity to look at a 9.1.3 Professional version on a collegue's PC.

First thing I checked: Is it really true, that Acroabat doesn't allow to edit OCR'd text?

Answer: No, not true. I could use Acrobat's builtin OCR engine to capture the text of a random scanned document which I google-searched and downloaded from the web. After that, this text was perfectly editable with the TouchUp Text Tool available via the Advanced Editing menu entry.

Procedure:

  1. Start Acrobat Professional; load your scanned PDF document.
  2. In the Document menu, click OCR Text Recognition and select Recognize Text Using OCR.
  3. Decide which pages you want to OCR in the Recognize Text window.
  4. Start the process and wait till it's completed.
  5. Now use the Tools menu, *Advanced Editing", and start the TouchUp Text Tool.
  6. From here you'll work it out yourself...
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Yes, that is more or less what I want to do. The result of the OCR process that Acrobat carries out is saved as a separate layer (after which it is just text, albeit hidden), and I would like to edit that layer. At this point, it makes little sense to keep referring to it as an OCR result, especially when comparing it to e.g, a PDF created from a word document, where the text layer is not an OCR result at all. –  Emil Jun 24 '10 at 22:06
    
This is an interesting question. I never needed to think about it, and I don't know enough. As soon as I have some time on hand I'll do some research (like study the relevant parts of the PDF spec) in order to find out more. Could well be that these hidden OCR'd text strings are made to be not editable at all. But maybe there is a workaround... –  Kurt Pfeifle Jun 25 '10 at 12:21
    
(After all, there are tons of OCR'd PDF documents out there. And OCR working with 99% accuracy is already regarded as "good". (But looking from the p.o.v. of a high school teacher, any text that has 10 spelling errors for each 1000 characters is earning one of the worst school grades you can imagine....) –  Kurt Pfeifle Jun 25 '10 at 12:22
    
There are programs that do this, but no freeware solutions. Regarding the quality of the OCR, that's not really relevant. In some situations, a single error, let's say a very embarrasing one or one that leads to serious misunderstandings, would be enough to make a solution to this necessary. –  Emil Jun 26 '10 at 17:30
    
This "answer" just confusingly restates the question. :-( You ask "Is your real question this" — yes, that's the real question, because that's what "text layer" means. And this is not an answer to the question. It's ok to post such answers before a clarification, but now please delete it, because it's a waste of time for those reading it. –  ShreevatsaR Aug 28 '10 at 19:23
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It appears what you mean by "text stream" is the text data from the PDF. Not sure. If that is the case, I use the standard clipboard and any text only editor, I use KEDIT because of it's column editing capabilities, to capture the data and edit it. The problem is that you lose any formatting with this and sometimes with tables the order of the data get messed up. But, for simple captures, it works.

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Yes, that is what I mean. I think I saw the term here somewhere and thought it apt. I'm afraid I wasn't clear enough. What I want is not to edit the text from the PDF, but in the PDF, i.e., to produce a PDF with a good, edited text part, so that a person copying and pasting from the document would get good and accurate text, while still displaying the document in its original form. –  Emil Jun 24 '10 at 19:02
    
PDF comes in more than one flavor. The Adobe version is "encrypted" and as such can't be edited without Adobe software. PDF is an open format that is defined in text which is editable by any text editor. Maybe someone can give you a reference to the PDF standard. –  Dave Jun 25 '10 at 0:30
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It seems rather far-reaching to say that Adobe pdf's can't be edited without Adobe software - certainly most third party PDF editing software manages to do that, or do you mean that they do it by reading the file and converting it into another format? I wasn't looking to write the program myself, so the standard will be of little use. Wikipedia (en.wikipedia.org/wiki/Pdf) has plenty of information and links for understanding the standard. –  Emil Jun 27 '10 at 14:24
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