export - File Size issue when exporting from Word 2013 to PDF

First off I hope I am posting this in the right community. If there is a more appropriate place please let me know.

My problem is this:

I regularly generate PDF files from Word documents which contains text and images. As long as I was using Word 2010, the behaviour of the export was fine, meaning that a Word .docx file of say 4MB would result in a smaller .pdf file of say 1MB.

But now that I was upgraded to Word 2013, the opposite happens! A Word 2013 .docx file of 500KB generates a .pdf file of 7MB, with all the same options as before...

I have tried to play with all of the export options I could find (e.g. "Delete cropped areas of images", pdf resoltution of 220, 150 or 96 ppi, etc.), but nothing seems to have any significant effect.

Constraints:

I have looked at some solutions from previous posts (e.g. Why is my PDF size from my InDesign Export still large?, Adobe PDF file too big to upload. What are my options?), but there were none that I can apply.

For example, I need to preserve the Bookmarks from the Word document, so Printing is not an option. I also don't have access to any of the other paid software sometimes mentionned (e.g. Illustrator, Photoshop, etc.).

Question:

Does anyone have any experience with Word 2013 and would know why the export to PDF has changed? Anyone knows of a way to reduce the size of exports to PDF while preserving the transfer of Bookmarks between .docx and .podf?

Thanks in advance for any help of tips you may have!

Vincent