Resize an electronic file

I had a Freelance artist create a book cover, size 8.5 x 11, for me in jpeg and pdf format. It will print okay, but when I upload it to Amazing Publishing, I get an error message, because the electronic files contain space on both sides. Amazon considers the file size to be 17.444 by 11.250 and gives me an error message. I need the electronic PDF file to be 8.5 x 11 with no right or left margins, which is what Amazon considers print ready. Thank you.