Best Workflow for compressing large, vector-based PDFs in Indesign?

Have you figured out how to? I've tried everything and I can't reduce them or rasterize them! 

Pixel images can be compressed by processing and simplifying groupings of pixels, but vectors can be difficult to compress because of the way they’re expressed as discrete objects. If the CAD files are “complex” because there are a high number of vector objects, you can try simplifying them in the CAD program, in Illustrator, or in Acrobat. By simplifying, I mean significantly reducing the number of objects by merging overlapping paths/shapes, reducing the number of points on paths, removing unneeded data types as Chad’s suggestion might do…those are the kinds of things that save space in a vector graphic.

I do understand that this is a portfolio, therefore preserving quality is a priority, but that conflicts greatly with keeping file size under 10MB. This question comes up a lot in video, too. People want to reduce the file down to email size, but are unwilling to accept the reduction in quality that would get that file down to that size.

Wanting to send a small file without compromising quality is totally understandable, but ultimately a major problem is often email itself:

• The file size limit on attachments can vary by provider, so just because it is below the sender’s limit does not necessarily mean it is below the limit of each recipient’s email system. 
• Issues with email encoding can mean a file is a lot larger as an attachment than it is on your computer. When an email attachment has to be translated to a plain text format during transmission, that conversion can make the attachment file size over 130% larger. If that larger size pushes it over the attachment file size limit on any sending or receiving system, the message fails. 

Because of these issues, and especially if this portfolio means you want to maintain a good impression by minimizing any hassles with blocked messages, it’s a lot more reliable to host a large file somewhere and send a link to it. Instead of spending a lot of time trying to crunch down the size, and then rolling the dice as to whether a portfolio still looks good enough and stays small enough to survive a trip through various email systems.

Optimize the PDF in Acrobat, as suggested by Chad, selecting all of the discard and clean-up options (unless there is something you specifically need), you may be able to wring some weight from the vector elements by simplifying in Illustrator and smoothing curves in Acrobat (keep an original copy), here is a link to a related discussion which explains in greater detail:

https://community.adobe.com/t5/indesign/exporting-a-pdf-with-both-psd-and-ai-files/m-p/11436640?page=1

"as PDF to a file size under 10MB"

Size does not matter: https://helpx.adobe.com/acrobat/using/sharing-pdfs.html

Why do you want to do this? I'm not trying to be a jerk, but it's entirely possible you are not being reasonable.

20 pages with any graphic content is going to be take up some storage space. If you can tell us the end game here we can offer much better advice.

If you export as "Interactive PDF" all vectors are already zipped.

Neither Acrobat Pro nor any other software will be able to compress them more because we do not know how to degrade the quality of vector objects by compressing them.

If you export as "Print PDF" be sure to check the "Compress text and vectors/artworks" option (I don't know the English item since I use a localized version).

Well the variable here is "several complex CAD files as PDFs". It will likely come down to getting the CAD > PDF file sizes down prior to placement in InDesign. How many is "several"? How "complex," and how were the PDF's generated?