I am self-publishing a book and need to change the dpi of all the photos from 72 to 300. I know how to do one photo, but the pdf is 218 pages. Is there a way to do all the photos at once (select all) or do I need to change them one at a time? If I have to change them one at a time, how do I save them so the file will stay intact? I'm computer-challenged, so if you answer my question, please give clear details and don't assume I know anything. Thank you very much.