How to remove anti-aliasing in PDF images?

I use Abbyy FineReader for ScanSnap to OCR a couple of scanned PDF files. The software claims it retains the original PDF images. The PDF file sizes pre-OCR and post-OCR are almost identical, which is good.

After the software is done, all PDF images appear anti-aliased in Acrobat X. Page navigation is much slower than before, and when I zoom in/out, the images first go to what looks like the pre-anti-aliasing version before quickly changing to anti-aliased images.

Left: Scanned PDF / Right: after OCR with Abbyy enter image description here

I would like to get the original images without anti-aliasing back. Interestingly, when I open a single page from the anti-aliased PDF in Photoshop, there is no anti-aliasing and the image looks like the left one.

My limited PDF programming experience leads me to believe that Abbyy likely sets some kind of anti-alias flag for each image during OCR processing. How do I un-set this flag?

Any pointers to useful ideas would be much appreciated.

Solution

There is /Interpolate true entry in image dictionary of OCR-ed version, and that's what causes 'anti-aliasing'. Whether that (and not JPEG2000 instead of JPEG compression) is a cause of slow-down, you check on large enough files.

To un-set this key, the best would be to turn it off while creating a file, and if that's not possible, to write and run a small program in suitable language.

But, since your file doesn't sport 'compressed objects' and offending key is in plain view inside a file, in the spirit of 'job done quickly' you can simply process your file e.g. like this:

perl -M-encoding -0777pe "s!/Interpolate true!' 'x17!ge" <in.pdf >out.pdf