Reducing file size

It might be helpful if we knew the actual resolution of the files as placed in InDesign.

Open either the INFO or the LINKS panel in InDesign, click on one of the images and it will show the Actual PPI and the Effective PPI. The Effective PPI is the important number.

In the image below you can see the actual PPI is only 72, however the image was scaled down in InDesign which raised the Effective PPI to 439.

Next, what are the settings you are using to export the PDF?

The final size of the PDF can vary according to the preset you select at the top of the dialog,

Click on Compression to show the settings. You can see that the PDF/X 1-a 2001 preset does less downsampling that the Smallest File Size preset.

Of course, the more you downsample the poorer the image, (more pixelation)

But its important to realize that the presets defaults allow for a much higher resolution before any downsampling occurs at all,

On the left you can see that a Color image will be downsampled to 300 ppi if and only if it is higher that 450 ppi

So the first image that was 439 PPI won't be downsampled at all using the default settings!

At default settings any color image below 449 ppi won't be downsampled.

Change the 2nd number to say, 310 ppi and any image above 310 ppi (rather than 450 ppi) will be downsampled to 300 ppi

You can also change the 1st number to downsample to for example; 260 ppi, 240 ppi, 225 ppi or whatever.

Click the images in the forum to see an enlarged view.

FYI:

You may want to do a "Save As" to your InDesign file.

That lower limit set by your client limits your options. Good images -> large file, bad images -> small file. You can't have both, you have to pick one of these.

(A similar choice which I'm facing almost daily is "Good, Cheap, or Fast: pick two out of three and forget about the remaining one".)