Huge File Size - Docx to PDF in Preview

I have word document (.docx on a mac) at 3.4 MB. When I save it to PDF it becomes a whomping 90 MB. I can't make it smaller and I can't figure out why it's happening. Here are the details:

1. I've tried to convert it to PDF through: save as pdf, print --> open as pdf --> save, print --> save as PDF.
2. I've done the PostScript work around and it did not help
3. I've used the quartz filter to reduce file size, it did not help
4. I've used websites to compress the PDF, they did not help
5. I've copied and pasted to a new word document, same thing happened.
6. The document has very few photos and things. It's 120 pages, mostly just text in Calibri, with a table of contents and page breaks. I am not annotating or altering the PDF in any way.
7. I have an old version of this file at 3 MB, PDF at 4 MB. When I open the old version and save it as PDF, it jumps to 75 MB. WHAT IS HAPPENING.
8. Other computers have had the same problem with it saving at 90 MB.

I do not have any adobe products, just Preview.

Even more than how to compress this file, what I really want to know is why is this happening? What is it about my computer or documents or preview that's making this so stupid??

THANK YOU.