Acrobat is not necessary. PDFs can be resized for free.
If you open a PDF in Preview and Export it you can choose the “Reduce File Size” Quartz filter. (Use “Export” not “Export as PDF…” in Preview.) This reduces size to between half to 1/3 of the original, in my experience.
To do this with a batch of files, another option for reducing PDF size is to use the “Reduce File Size” filter in the “Apply Quartz Filter to PDF Documents” action in Automator. Make sure your Automator workflow is making a backup saftety copy so that the process doesn’t trash the original. Delete the backup after you’ve examined the result.