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I use the PDF Optimizer feature in Acrobat Pro (v8) to reduce the file size of DTP-generated PDFs by downsampling the embedded images to a lower resolution. This normally works fine.

However today I've tried the usual profile on another image-heavy document. The PDF size is 350MB and according to the Audit Space Usage the images make up 99.64% of that. But when I try running PDF Optimizer the file size just does not change.

I've tried with Optimizer settings to get the images down to 72dpi but the file size stays (almost) exactly the same. I say almost because the size does reduce by a couple of MBs but that's from other settings in the PDF Optimizer.

Could this be because the images embedded in the PDF are PNGs rather than JPG?
This particular PDF has come from an Inkscape SVG which I'm pretty sure used high-res embedded PNGs. I could understand if the JPEG compression settings were ignored for embedded PNGs but surely they should still be downsampled. Also I've got hidden images set to discard, layers flattened and images merged, none of which make any difference.

Anyone had this before and knows of a way to convert the embedded PNGs to JPEG in Acrobat?

I'm thinking my best option might be to convert each page of the document to EPS, then convert all those back to PDF and then optimize. Hardly ideal. And the output doesn't exactly match the original PDF, with vector elements and text occasionally chopped around.

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Maybe your file is already optimized ?? – Searush Nov 7 '12 at 14:45
I read that sometimes sending it through the Adobe PDF Printer works to make it smaller? – j_bombay Nov 7 '12 at 14:47
Aha. It took an age to print it through but that brought the size down to what I would expect! But it rasterized the text. The Adobe PDF Printer seems to use the settings from Distiller but Distiller doesn't support converting PDFs directly. It might be time for me to upgrade to a newer version of Acrobat as this seems to be a known issue with PNGs embedded in PDFs. – batfastad Nov 7 '12 at 18:21

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