Welcome to Android Central! Are you saying you no longer see Downloads in the App Drawer? It's standard in all versions of Android--you can check to make sure it's still around by going to Settings>Apps>All, and looking for Downloads and Download Manager.
You can always access your Download folder by using a file manager app like ES File Explorer or Total Commander.
You might be able to clear Polaris as the default PDF app by going to Settings>Apps, selecting Polaris, and tapping Clear Defaults. If there are no other apps that can open PDFs, you might not be able to clear the deafult, though. It's possible that the previous PDF viewer system app may have been deleted as part of the system update, with the intention of making Polaris the default.
Finally, this experience can serve as a reminder to always back up your data before a system update!