Exporting PDF with black and white photos- WHY IS IT SO LARGE?

[azurm22632908]

I have had this issue for over a year now. I do a lot of "treatments" for photo clients. Basically indesign docs of full spread images with a little type here and there, exported as PDFs. When I make these it is always easy to export them and get the pdf file size somewhere around or under 12mb with some compression and color space adjustment.... UNLESS! Unless the imagery I'm using is black and white! Its not greyscale, its an RGB image but with black and white adjustment from lightroom and then exported as a tiff or jpg. If I were to make two identical indesign files and export two pdfs with the exact same settings, one with color RGB images and one with black and white RGB images one, the color would read 13mb and the black and white would read 85mb. WHAT AM I DOING WRONG HERE?? My only poor solution so far is  exporting a pdf and then running it thought acrobat to downsize it again, just to get the file slightly smaller but its still usually well over 12mb. PLEASE HELP

[Barbara Ash]

What settings are you using when creating the PDF? Are you using PDF/x-4:2010?

[azurm22632908]

Yes I’ve tried that export setting. As well as “smallest possible file” and custom settings, where I downsize to 72dpi and set the quality to “low”. I’ve tried converting the color space, keeping the profile, discarding the profile, setting to to CMYK, sRGB. I swear I’ve tried everything.

[Barbara Ash]

I tried exporting to PDF from InDesign with just one image, one color and one RGB black and white. The PDF with the black and white image was slightly larger, only half a percent larger, no where near the huge difference that you are seeing.

Does this happen with all files or just one?

[azurm22632908]

It happens everytime I make a pdf with black and white images. Ive never tried just one image... but with over 10 full spread images i guess that starts to add up? 

[Barbara Ash]

I don't think it shouldn't be adding up to anywhere near that much difference, even with 10 full page spreads.

If you're able to upload an example PDF with color and without color I'll take a look at them.

[Willi Adelberger]

I am confused of your description.

The correct way would be with complete B&W imagery: Save images as Greyscale and export them as CMYK with Convert to CMYK with keep numbers to avoid converting them to 4c images.

The file size is given by the size in pixels and the embedded output color profile.

You can also use a greyscale output profile in your case if the font has no other color neither. 12mb would not be so extraordinary large if it is a print file.

[mlyn]

This is a helpful response. I found this community post when I encountered the same issue. I created a color poster and saved a 1.2mb PDF, then decided to do a black & white version. I changed all the text to Black and updated the image file, and the exported PDF was over 5mb. It had LESS color and was 3x heavier. I tried all sorts of alternatives with the image and nothing about the PDF size budged. Finally I created a grayscale, flattened JPG (not PSD) and linked that to the b&w poster file, followed the guidance here when exporting the PDF, and got a 2mb file. I'm still baffled how a file with less color information is heavier, but at least it's not as bad.

[ahBiscuits]

The only solution I've found to work is to use colour images but overlay a white block and setting it to 'colour' as the transparency blend mode.

The result for me was to be able to create a digital brochure pdf that was 2mb instead of 23mb.

[Test Screen Name]

It would be interesting to see a pair of brochures, directly out of InDesign (not shrunk again), one colour and one greyscale, showing the big size difference. Since this doesn't happen to everyone there may be a specific cause that could be found with a detailed analysis.

[mlyn]

I'm attaching the color original and a version of the black & white attempt without the fixes, so the file sizes are as I described in my comment above.

[Peter Spier]

I'm no PDF expert, but there are definite differences in how the image is described in the preflight data.

It looks to me like the "B&W" version of the photo contains a whole lot of additional data required to render the RGB file as a grayscale indexed color image, so it has not only the RGB values, but a matrix of additonal values for output. I think this is a direct result of using the black and white adjustment layer and neither flattening nor converting to grayscale.

I wonder what the size difference is if you use the version with the adjustment layer for both posters, but hide the layer visibilty for color output. I suspect it is going to be similar.

[mlyn]

Informative, thank you! Flattening it and converting to a grayscale JPG helped, so that supports your theory. I couldn't get preflight to work on Acrobat DC so I appreciate the extra set of eyes.

[Test Screen Name]

My analysis:

The colour image is stored using JPEG compression; it's about 4% of the original size.

The "greyscale" image did not land in InDesign as greyscale at all, but as a colour image (which happens to have only 256 shades of grey). Now, InDesign has analysed this, and found it is a colour image with less than 256 colours. This is treated with great caution: it may be a screen shot, or a line drawing. So, after some transformations to make it smaller, it is compressed using only ZIP compression. As a result, instead of being 4% of the original size, it is 63%. 

This image was treated as sRGB by InDesign. You need to get it into InDesign as actual greyscale. How you do that, I leave to others...

[mlyn]

Thank you. Helpful to know that InDesign is looking at the original image file with all its layers and profiles, not just treating it as the visible pixels. Once I converted the image to a flat grayscale JPG, the PDF was much smaller.