Preview not opening scanned files.

If rebooting your Mac doesn’t fix things, see if the problem still happens in Safe mode (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode disables loading third-party drivers, forces the OS to do disk repair, clear certain caches and logs and other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

If the problem is still unresolved, please return here and post a reply and we’ll see what else we can come up with for help.

Good to know that it worked in safe mode.

Give this a shot –

Try resetting the NVRAM. The NVRAM is a special memory section on your Mac that stores data that persists even when the Mac is shut off, such as volume settings and screen resolution - it's the information the computer uses to maintain settings after a reboot. Best to hold the keys at startup until the Mac has chimed 3-4 times, then release.

Reset the NVRAM

How to Reset NVRAM / PRAM on your Mac - Apple Support

If that doesn't work, see if the problem exists only in your computer user account or across all users. To test this you can easily create a new user by navigating to System Preferences > Users & Groups.

Uninstalling Preview is not an option since it's an integral part of macOS. However, it is possible to reinstall your current version of macOS and all of it's components without wiping out any of your user data.

Reinstalling macOS does not have to erase your user data.  The only time that should happen is if you choose to delete, erase, format or partition your startup drive. You won't do that at this time.

If you select Reinstall macOS High Sierra from the macOS Utilities window after rebooting in Recovery mode, the OS should be reinstalled on top of the current installation, thereby refreshing any missing or damaged files within the current macOS version. That is assuming that the drive device and volumes are healthy.

As always, it is recommended to back up your Mac before attempting this... just in case.

Back up your Mac with Time Machine - Apple Support

To non-destructively reinstall macOS as a troubleshooting measure - without deleting user files, documents, pics and vids, etc. – boot your Mac in Recovery mode by restarting and when you hear the startup chime press the Command-R (⌘R) keys until you see the Apple logo and progress bar.

Follow the onscreen instructions to “Reinstall macOS…”.

How to reinstall macOS - Apple Support

NOTE– You should not have to delete, erase, format, partition or otherwise alter your startup disk for this. If that becomes an option, quit the process and restart your Mac normally. Then come back here for additional advice.