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I am using Adobe Acrobat XI Pro to work with PDF documents, many of which I download from Google Books and read on my iPad. However, some of the Google Books PDFs are very large and slow to load on the iPad so I have been trying to reduce their size, with a combination of using Acrobat's OCR, save with reduced size and save optimized options. However, with some of the PDF files (and in reality the ones which are hardest to work with) I constantly get this error message: "The PDF document contained image masks that were not downsampled." This means, it seems, that the images' resolution cannot be reduced - which results in the fact that I cannot make the files any smaller.

How can I make it so that Acrobat can reduce the file size? I have tried to extract each page individually, shrink that, and recombine, but it doesn't see to work.

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Image masks are often used when applying transparency which Acrobat might not be downsampling due to the risk of causing a visual change to your document. It is possible to flatten transparency (which again may cause an unwanted appearance for the image) using the Preflight tool in Acrobat Pro.

  • Acrobat XI Pro > Tools > Print Production > Preflight > PDF Fixups > Flatten transparency

I'd suggest trying this to see if the appearance of the images is acceptable to you after doing this as it should reduce the file size. Some common Word processors and similar applications use transparent images when it is not necessary so hopefully that is the case for you.

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I had an 88 page PDF file over 40MB and I got the same message you got when I tried to reduce the file size. Here is what I did: Instead of saving it to reduced file size, I printed it to PDF, which brought down the size to 22MB. Then I took that file and it let me reduce the size the regular way to 16MB.

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This isn't an expert answer, but this is what I've found. Even though I get the error message, I go ahead and click SAVE. The file size is reduced by almost half and all the pages still look great.

  • Can you maybe give a link to an image or something to show that this works? – DrZoo Dec 16 '16 at 2:28

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