How do I reduce the trim size of a multiple pdf file in adobe illustrator?

Does your book have bleed? If so, the overall size of the PDF will necessarily be larger than 8.5 x 8.5. In Acrobat, go to Tools> Print Production> Output Preview, do you see the little check box Show art, trim & Bleed boxes? this will show you the invisible bounding boxes. You can check the trim size by clicking the button Set Page Boxes and Apply to: (Trimbox) the size of the trim box will be indicated under the page preview.

If you do need to re-size your PDF, this can be done in Acrobat using a Preflight profile, or by placing the PDF into an InDesign file of the desired size (using the place multipage PDF script) and exporting to a new PDF.

If you do have the original source file as Monika has suggested then this should help you understand what you need to do: https://helpx.adobe.com/illustrator/how-to/artboards-basics.html

It is not advisable to import a PDF in Illustrator.

If the source file has been made in Illustrator, get the source file and then open it.

For this kind of operations, use Acrobat.

https://helpx.adobe.com/acrobat/using/printer-marks-hairlines-acrobat-pro.html

Hi in adobe illustraor after opening file you can use artboard tool to resize the page size....regards