If you can't open files saved in your Dropbox on the Android app, it could be because the file is corrupted. Try viewing the file in a third-party application to find out if there's an issue with the actual file:
If you cannot view the file in another app, your file may be corrupted. If this is the case, you will not be able to view the file in the Dropbox app.
The Dropbox Android app requires an internet connection to open, preview, and upload or download files. If you can't complete one of these actions, verify that you have a strong WiFi or data connection and try reloading your file.
If you are connected to internet and still having issues, try signing out and signing back in:
You may encounter issues downloading large files from the Dropbox app on your mobile device. Unfortunately our mobile clients do not have a "resume download" feature, so large files (usually those larger than 100MB) may not download properly. This is especially true over a mobile connection, where connectivity can be intermittent.
If an upload is failing on your mobile device, you can try uploading the file via the Dropbox desktop application on your computer. The Dropbox desktop application can handle files and folders of any size and any amount of files and folders. If you do not have Dropbox installed on your computer, you can download the application and sign into your Dropbox account.
Caching issues can be cleared by reinstalling the Dropbox Android app, but you can also manually clear your cache: