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I am trying to create a PDF document that includes many images. All of them are on the foreground except one, which is a monochromatic background and spans the whole document area. I found that without the background, the PDF file size (single page) is around 10MB, but with the background, the file size can go to 20-30MB.

What I would like to do is to reduce the PDF file size. Since the foreground images need a high resolution (~300dpi), I don't think I can do much about them, so I would like to ask if there is anything I can do to the background image before or after creating the PDF. I tried to reduce the resolution of the source image before adding to the document and converting to PDF, but the reduction in PDF file size is quite minimal. I wonder if I can take any advantage of the monochromatic nature of the background image, or whether I can use some software to selectively optimize the background after the PDF is created. Thanks!

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You don't mention what you're using to do your layout.

If you have Adobe Acrobat Pro, and in the way you made your graphics, you could selectively lower the resolution of the grayscale or color images only. But for this, your image must be in a real grayscale mode.

If you have Adobe Acrobat Pro, open your file with it.

Go on the file menu and select "save as other...", and then "optimized PDF"

On the "image" section, you can lower each mode of image (color, grayscale and monochromatic 1bit).

If you already know you need a resolution of at least 300dpi for your images, might as well optimize the color images as well. You can use the value 300dpi for color, 300 to 600 dpi for grayscale and 600 to 1240dpi for monochromes. You can use the zip compression for all of them, and if you want more compression, you can use the "maximum JPG" compression.

If you really only want to optimize the grayscale, simply leave the other color to the "off" for downsampling and only downsample the grayscale.

If your PDF doesn't need any other functions such as hyperlinks or special scripts (eg. fields), you can safely check all the checboxes in the other sections. That will flatten your artwork, remove private data that are stored in the PDF (eg. profiles, user names, preferences, etc.) and will also crop the images in your PDF to the right size.

if you need any of these features, you'll need to be careful and look what each checkbox does, and make some tests to see for yourself.

If you used transparency, make sure you put that setting to high resolution.

After this, you can save the PDF. Save it to another name in case you don't like the settings you used to optimize it and need to start over.

This should really lower the size of your PDF file.

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  • Thanks, your answer makes much sense. The only issue is that my background is monochromatic (i.e. shades of the same color such as blue), but not necessary different shades of grey. So it seems impossible for my background to be saved as grayscale.
    – GreenPenguin
    Commented Jun 22, 2015 at 10:02
  • Indeed, you're right. Maybe you can try optimizing everything but as you said, the background solution only won't work. It's probably worth it even if you keep the same resolution. It really makes a noticeable change in the size even without lowering any resolution and keeping everything at 300dpi.
    – go-junta
    Commented Jun 22, 2015 at 10:07

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