If my self published book is in PDF form, can I just download the file to create an ebook?

PattiPaige2020 wrote:

Thank you. Will jpegs work?

I'm not sure what you mean by that. Apple does not accept books in .jpeg format, only in .epub format (or .iBooks format, which requires making them with the iBooks Author app).

https://itunespartner.apple.com/books/articles/book-formats-2708

Apple's Book store does not accept pdf. You will have to convert your pdf by some means into the .epub format.