Hey Regan, to reset you Chrome Selfie X, go to Settings > Backup and Reset > Factory data reset > Reset Phone... Do please kindly note the following though.
1. Your downloads may not open because they are in formats that your currently installed applications do not support. To overcome this, simply check what each of your downloads are in and then head to Google Play Store and download an app that is dedicated to opening the same. An example would be to look for a PDF viewer app for a downloaded PDF file.
2. It is necessary to go to Settings > Security > ... check the "Unknown sources" box before you can install non-play store applications (apk)
3. It is otherwise recommended to download applications directly from the Play Store as those are guaranteed to work for your device.