Hi Alexandra. If you can let me know what browser you are using, I can probably help. It's important to make sure your browser is up to date, but some have also recently decided to open all PDF in their own in-browser reader which wreaks havoc with multimedia files.
It is best to view the document in Adobe Acrobat reader, rather than a third-party or browser reader.
So, I can also suggest downloading the file rather than viewing it in a browser.
Let me know if any of that helps. Chrome is particularly stubborn on this issue.