I think if you use Illustrator or coreldraw and export as a PDF, the file size should be minimal under 2MB. If you have images of a higher resolution in the design, I'd suggest you try to reduce its quality to the total file size can be under 2MB.
If build a reputation on the platform, it will get a certain stage fiverr will allow you create a portfolio on your profile where you can include your case studies.
I hope this helps.