Reducing the generated PDF size / compression PDF

[Sébastien Cuendet]

Hello everyone,

[I apologize for double-posting, but it seems that my first post was not published]

The setup of my (web) app is the following: I get user uploaded PDF files, I run OCR on them and show them the OCRed PDF. Since everything is online, the minimizing the size of the resulting PDF file is key to reduce loading and wait time for the user.

The file I receive from the user is sample.pdf (attached to this post). I use tesseract 3.04 and do the following:

gs -r300 -sDEVICE=tiff24nc -dBATCH -dNOPAUSE -sOutputFile=sample.tiff sample.pdf
tesseract sample.tiff sample-tess -l fra -psm 1 pdf

The result of the OCR is good, but the size of the generated PDF is now about 2.5 times as much

- size of original pdf file: 60k

- size of final pdf: 147K

So I ask you, how can I reduce the size of the generated PDF while keeping the OCR result?

One obvious solution is to reduce the resolution when generating the tiff, but I don’t want to do that as it may affect the OCR result. 

The second thing I tried was to reduce the PDF size post-tesseract, using ghostscript:

gs -o sample-down-300.pdf   -sDEVICE=pdfwrite   -dDownsampleColorImages=true   -dDownsampleGrayImages=true   -dDownsampleMonoImages=true   -dColorImageResolution=300   -dGrayImageResolution=300   -dMonoImageResolution=300   -dColorImageDownsampleThreshold=1.0   -dGrayImageDownsampleThreshold=1.5   -dMonoImageDownsampleThreshold=1.0 sample-tess.pdf

This helps a bit, the generated file is only 101K, so about 1.5 times the original. I could live with that, but it also seems to affect the OCR result. For example, the white space between ‘RESTAURANT’ and ‘PIZZERIA’ (second line) is now missing.

Another (simpler) option with ghostscript, using the ebook parameter, results in a 43k file with some lesser quality in the PDF and the same problem of the missing white spaces:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dBATCH  -dQUIET -sOutputFile=sample-ebook.pdf sample-tess.pdf

The lesser quality of the PDF is fine, but again, I don’t really want to compromise on the OCR.

I’ve done other tests using PNG and JPEGs, but the OCR results always go down (even slightly) and the resulting PDF is not smaller. For example, with PNG:

convert -density 300 sample.pdf -transparent white sample.png

tesseract sample.png sample-tess-png -l fra -psm 1 pdf

The total (55.50) is missing and the final pdf size is 149k.

So to summarize, here are my questions:

• Can someone explain why reducing the size of the PDF using ghostscript affects the OCR result? I thought the text layer and the image layer were independent… 
  
• Are there options that one can give to tesseract to reduce the quality of the images when it generates the PDF?
  
• I read that other solutions like ABBYY OCR use Mixed Rasterized Content (MRC) to reduce the file size. Does tesseract do that already? If not, are there some open source or proprietary CLI tools that do that, which I could use to reduce the tesseract generated PDF file?
  

Again, I’m OK compromising on the quality of the PDF images (although I would like to keep the colors, ideally) as long as the user can search text and select it to copy/paste from the PDF.

Any help would be greatly appreciated!

[ShreeDevi Kumar]

You could also test with

gswin32c -q -dNOPAUSE -dBATCH -sDEVICE=tiffgray -sCompression=lzw -r300   

ShreeDevi
____________________________________________________________
भजन - कीर्तन - आरती @ http://bhajans.ramparivar.com

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[Sébastien Cuendet]

Thanks for your answer.

I did try that as well, but I lose the colors and the final tesseract file is still 142k (only 5k less than the one with colors), so not much of an improvement.