Mark,
I answered a question similar to this a while back and it turned out that the publisher was looking for a banner ad versus a PDF for download.
If they are looking for a PDF and specifically state 72dpi, it is likely that they just want to ensure a small file size. David’s response of using Smallest File Size while upping the compression quality usually does the trick.
PDFs can be made up of vector artwork and also raster artwork of various resolutions. The export will rescale color, grayscale, and monochrome images based upon the settings. For example:
All of the presets with the exception of Smallest File Size use the following settings:
Color Images: Scale any image over 450ppi to 300ppi
Grayscale Images: Scale any image over 450ppi to 300ppi
Monochrome Images: Scale any image over 1800ppi to 1200ppi
Smallest File Size uses the following.
Color Images: Scale any image over 225ppi to 150ppi
Grayscale Images: Scale any image over 225ppi to 150ppi
Monochrome Images: Scale any image over 450ppi to 300ppi
In addition, it also converts the document to sRGB
You could create your own custom export that would resize any image over 72ppi to 72ppi. However, you will start to see noticeable degradation in the quality. Smallest File Size with slightly better compression is the safe bet.
If the publisher is looking for a web banner, you may want to check to see what the desired raster image format will be (e.g., jpg, png)
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