OK - if you have a good backup of the machine from before the upgrade, this is very much not a straightforward process and at worst, you'll need a second backup drive (or space on Time Machine if you use that as your backup.) or lose your backup when it's the only copy of your data. This is one case where I would probably make a backup copy of the one backup interval until I'm sure going back to Sierra worked as well as a perfect bootable clone of Mojave if your Sierra is broken when you go back to it. Mojave may have brought to light the broken user account / data and not have been the cause of your problems with Preview, etc... Consider if this makes things even worse before you start.
Now, this isn't automatic as Tetsujin points out - data formats change permanently and irreversibly in some cases when you upgrade so you can't roll back the upgrade and have all your data roll back as well.
- You lose all the work you made since upgrading unless you manually export and import that.
If you don't have a backup, the erase and reinstall is easy - but you'll have to hand migrate apps and data back and possible re-install things that broke and might even lose an app or two if you can't find a working installer to run now.
I'll assume you use Time Machine and have space for all the Sierra backups and one good Mojave backup and you don't mind losing everything if you make a mistake:
- Perform one last Time Machine backup of the Mojave so you can get any files you modified in the time between before the upgrade and now. You are about to erase everything so you need this backup if the
- Organize yourself - think about which data really lives on the Mac. If contacts and photos are in iCloud - consider turning that off so your backup / restore is smaller. Get the exact date of the backup you choose to restore to - verify that the things you need are there. Also - make a list of the key apps.
- Make sure you have the Sierra installer - see this to get that installer. How can I download an older version of OS X/macOS?
- Follow the ease install procedure - https://support.apple.com/en-us/HT204904 - noting you have to bring the Sierra installer if Internet recovery doesn't let you select that version easily.
- When Sierra boots, connect your backup you identified above as the "pre Mojave" and get all your apps and data back.
- Validate that the iCloud / off computer data is syncing again (google mail and calendars, iCloud, whatever else)
- Then go get any documents you added from the Mojave backup
- Remember why backups are so important and why computers where you can revert things are awesome. This wouldn't be possible on iOS and other "always forward" platforms.