I have a bunch of PDF files that came from scanned documents. The files contain a mix of images and text. Some were scanned as images with no OCR, so each PDF page is one large image, even where the whole page is entirely text. Others were scanned with OCR and contain images and searchable text where text is present. In many cases even words in the images were made searchable.
I want to make an automated process to recognize the text in all of the scanned documents using OCR, with Acrobat 8 Pro, but I don't want to re-OCR the files that have already been through the OCR process in the past. Does anyone know if there is a way to tell which ones contain only images, and which ones already contain searchable text?
I'm planning on doing this in C# or VB.NET but I don't think being able to tell the two kinds of files apart is language dependent.
Use "dtsearch" to create an index for all the pdf files... then "view the log file" of the indexing process to check the list of pdf files that were not indexed.
After opening the PDF, try searching for a word known to be in the document (preferably a word that appears on several different pages) by clicking CTRL-F and entering the word in the Find box. If the message below appears, the document is not text-searchable.
Scannned images converted to PDF which have been OCR'ed in the aftermath to make text searchable do normally contain the text parts rendered as "invisible". So what you see on screen (or on paper when printed) is still the original image. But when you search successfully, you get the hits highlighted that are on the invisible text.
I'd recommend you to look at the XPDF-derived commandline tools pdffonts(.exe)
, pdfinfo(.exe)
and pdftotext(.exe)
. See here for downloads: http://www.foolabs.com/xpdf/download.html
Example usage of pdffonts
:
C:\downloads\> pdffonts cisco-ip-phone-7911-guide6.1.pdf
name type emb sub uni object ID
------------------------------------ ----------------- --- --- --- ---------
LGOKFL+Univers-BlackOblique Type 1C yes yes no 13171 0
LGOKGM+Univers-Black Type 1C yes yes no 13172 0
[....]
This PDF uses fonts (indicated by the 'name' column), has them embedded (indicated by the 'yes' in the 'emb' column) and uses subset fonts (indicated by the 'yes' in the 'sub' column).
C:\downloads\> pdffonts examle1.pdf
name type emb sub uni object ID
------------------------------------ ----------------- --- --- --- ---------
Univers-BlackOblique Type 1C yes no no 14 0
Arial TrueType no no no 15 0
This PDF uses 2 fonts (indicated by the 'name' column). The font 'Universe-BlackOblique' is embedded completely (indicated by the 'yes' in the 'emb' column and the 'no' in the 'sub' column). The font 'Arial' is also used, but is not embedded.
C:\downloads\> pdffonts examle2.pdf
name type emb sub uni object ID
------------------------------------ ----------------- --- --- --- ---------
This PDF uses not a single font, and hence does not have any text embedded (so no OCR either).
Example usage of pdftotext
:
C:\downloads\> pdftotext ^
-layout ^
cisco-ip-phone-7911-guide6.1.pdf ^
cisco-ip-phone-7911-guide6.1.txt
This will extract all text strings from the PDF (trying to preserve some resemblance of the original layout). If there is no text in the PDF, you'd know there was no OCR...
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