TurboTax Online can only output PDF files. The file may be corrupt because it’s not downloading correctly.
Try clearing you cache, or using a different browser, and then re-download the PDF file.
Here is how to clear your cache (browsing history): https://turbotax.community.intuit.ca/replies/2647843
When it gives you the link to your PDF, don’t click directly on it. Instead, Right-Click the link and choose “Save Link As”, and save it to your desktop or wherever it’s easy for you to find. Make sure that when it saves it is saving as Your-Name-archive-cra-18.pdf and not octet-stream or something weird like that.