As shown in the screenshot the "Adjust Size" is disabled and grayed-out for the given PDF:
So how do we reduce the size of a pdf in preview (or even skim?)
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The Adjust Size / Colour feature in Preview doesn't work for PDF files, only for bitmap image files. Usually you don't want to change the format of a PDF document, anyway.
However, if you have a PDF file that contains only embedded bitmap images, and you want a downsized version of that image, it's possible to use the Preview's export feature: File > Export. The export dialog will ask for a pixel density (in ppi) to use for the export, as it is assumed that you are converting vector graphics/text to bitmap graphics…
Another possibility is to use the marquee/selection tool to select an area or the whole page and press Cmd-C, then create a new Preview document from the selected area by pressing Cmd-N, and save the new document in a bitmap format.
I can't say what it does to the exported image if you put in a ppi value different from the actual ppi value of the embedded image. There are commercially sold third-party tools that extract bitmap images from PDFs without resampling them, e.g. Acrobat Pro/DC and others. A smaller and free tool is downloadable via SourceForge: https://sourceforge.net/projects/pdf-images/
The following approach worked for me in Mac OS X 10.13 (High Sierra):
You don't have any control on scaling parameters, but results were good enough for what I needed.