If you’re using a Mac, make sure that it is not “Preview” that opens it, but Adobe Reader. Look at the top left hand corner of the screen and it will say either Preview or Adobe Reader. As long as it says Adobe Reader, you’ll be able to insert all of your own photos and add your own text to the flyer.
To open the PDF in Adobe Reader, right click on the file and select “Open with” and select Adobe Reader. If you don’t have adobe reader, you can download it for free here: https://get.adobe.com/reader/