Re: I created file in Illustrator and saved as PDF. It was 15 mb, which is fine. I then went in and corrected some text and added a paragraph of text and saved as PDF, again, and the file size jumped to 900 mb. What did I do wrong on this second go aroun

Sounds like there were photos in the PDF.

You might want to save 2 files: one AI file (in case you need to ever edit it again)

One PDF file without Illustrator editability. Should be much smaller.

Also check compression settings.