what size does the pdf file reduce to in preview to export?

It depends on the PDF input. Images may not compress very much or it may alter the quality in a bad way.

Just export it using the reduce file size filter & save it using a new name - if the size is too large or the quality is not good enough you will need to consider a different approach.

Highlite your file in question, Command i will open the info pane—here you can see for yourself the file size.

PDF are typically vector files and can maintain high quality (especially for printing) without being a large size, compared to a rastor file (like a jpg).

If you print via command P--> from the PDF dropdown you have options:

There is another way to control the reduction in size of PDF's by changing the "image compression" from within ColorSync Utility - the existing filters are locked but you can duplicate and give it a different name, then you open your PDF with ColorSync and select your "new" filter.

The standard filter options,

Click on the right hand side arrow of "Reduce File Size" and select "Duplicate Filter",

You will then get "Reduce File Size Copy", click it and rename - to save press the enter key,

I've renamed mine "PDF Copy",

Then click on the left arrow to access the settings,

It's "Image Compression" you will need to experiment with but the nice thing is that it can be set at any point between min to max, to use the filter you then open the PDF with ColorSync using "Open With",

You select your filter from the menu on the left and then select "Apply" on the right, once processed you can then save the file.

If you are not happy with the size/quality, you go back to your ColorSync filter and alter the quality slider and process the file again - there is a "Live Update from Filter Inspector" option which I think allows you to see in real time the effect of moving that slider, but I've not tried that yet.

Hope this helps.

I don't think there is an easy way to control the filter within Preview's export options.

Automator has the ability to edit the settings within the filter, however I suspect you will get better results via a specialist application for generating PDF's. You may also consider optimising the source images in the application that generated the PDF - you haven't really given us much info to help with that though.

What kind of book is it for? Is it an ebook or something for an actual printing press with hard copies? If it is going to be printed I'd forget about compressing it at all - that always throws away quality in exchange for file size. You are certain it needs to be smaller?

In Automator you create a workflow, add the 'Apply quartz filter to PDF' action & then tweak the settings & keep making PDF's until you have one that has sufficient size & quality but it is a repetitive process.

This might help https://computers.tutsplus.com/tutorials/how-to-reduce-the-size-of-your-pdf-with out-compromising-image-quality--cms-2169…

It talks about employing custom quartz filters to control how much compression with respect to quality loss.