Hi @kazzam 
Welcome to the Canvas Community. This is only a guess, but maybe it is a pop-up blocker. While I'm not positive that that Canvas DocViewer is treated as a pop-up for the browser, it's behavior is certainly similar enough to one to cause me to think that may be it. Check over this online lesson: How do I allow pop-ups for Canvas in my browser? If that does not help, head here: Which browsers does Canvas support? and not only check that your browser is updated, but also look over what that page mentions about certain security settings. And finally, just as a precaution, clear your browser's cache, as cover here: How do I clear my browser cache on a PC? and here: https://community.canvaslms.com/docs/DOC-10708-67952720330 Browsers (and security settings) are ever-changing, and hopefully one these helps, Khoula! In the meantime, you can always download files by right-clicking over the link and then Save link as... (or the similarly-worded option on your browser.)
