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I have one paper scanned into a PDF, which has a size of about 700KB and 300dpi. But if I take a snapshot of the PDF file using DPI settings of 300 as well, the image file (in PNG, JPG, or TIFF) is more than 2.5mb, even if after I tried several optimization tools. If I extract the image using pdfimages, the result is even bigger, about two 50mb PPM files, which can be converted into two 1mb PNG files.

My question is, how does the original PDF file achieve so high compression level? The metadata says it is created by Adobe Acrobat 7.0, PDFScanLib 1.2.2. And is it possible to achieve such compression level with other software? I need to manipulate the original PDF, but any attempts now substantially increases the file size or decreases its quality.

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How what the original document scanned into the PDF? Is it just text, or are there images as well? Is the text "Text" (ie: you can select it), or is it just a scanned picture of the Text? Have you tried compressing your snapshot versions? Try Zipping or RARing or 7z'ing a couple of them and see how it compares. Have you read this yet? –  techie007 Dec 22 '12 at 17:20
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One would need the actual PDF to answer this question. –  Dour High Arch Dec 22 '12 at 18:00
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When you edit the picture and save as JPG, most picture editors (like gimp) allow you to select the quality level. For JPG you can select range from 50% to 80% to see if it fits your needs. You can also resample down to 2x2x2 color scheme when saving.

More importantly if your paper is text, plain graph, or screenshots, then most editors allow you to simplify the picture down to 256 color, and save as GIF or PNG.

In this way I have been able to get very small file sizes.

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The question is about PDFs, not JPG, GIF, or PNG. –  Dour High Arch Dec 22 '12 at 19:24
    
He is complaining about "the image file (in PNG, JPG, or TIFF) is more than 2.5mb". –  user182109 Dec 23 '12 at 10:11
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