How to reduce PDF file size from Illustrator?

I am a graphic designer using Adobe Illustrator CC and Acrobat Pro DC. I am using Save As command to create pdf files from Illustrator. I then use Acrobat's Combine Files into a Single PDF command to create a Binder from the pdfs I have saved. I need to save these pdfs from ten fairly large (25-50MB) Illustrator files and have the total file size be 10 MB or less.

So far, the smallest file size I can create is around 28MB. I have tried using the Reduce File Size and Optimize command without any success. I am using revised versions of files I created before the last CC update which I didn't have any trouble reducing to less than 10MB.

I would like to create a Preset with the Acrobat settings which would give me the best result. Any help would be greatly appreciated.