Why is my Scrivener file so large?

I have a 100,000 word novel complete with lots of research in one Scrivener file and it’s about 500 MB. I started a new novel file with about 5 pdfs and it’s almost 1 GB. Can someone help me figure out why this new file is so large? It’s taking forever to load on Dropbox/thumb drive.

How large are the PDFs? The quantity of them does not really matter since one PDF can be 85 gigabytes large, or 100 kilobytes.