Preview doesn't save to pdf properly.

Try the following: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Also have you installed or run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

We are all fellow users here, save for the occasional Apple community specialist assigned to the Apple community host team.

I have macOS 11.4 (20F21) on an M1 mini, and I have absolutely no issues with multiple Preview sessions on the same or different PDF losing prior annotations (my term for edits). My PDFs are opened and saved to the local boot drive, not Apple's or another cloud service, or not to a Windows server share. I do not run any anti-virus product, or use any so-called cleaner applications that may harm application internals. Not insinuating that you do, but stating what I don't do…

You may have some twisted cache, or Preview configuration data causing your issues. If it was a bug, I would have it too.

What you can try, with no promise it will fix Preview, are the following:

1. Boot into Safe Boot mode, and then reboot normally, and then see if the Preview issues persist. Safe Boot is doing several housekeeping chores and will take much longer in the boot cycle. There may be some harmless screen artifacts too.
2. In your /Users/username/Library folder, after quitting Preview
   1. Look in the Caches folder for anything related to Preview and move it to the Trash.
   2. In the Application Support folder, locate the Preview folder and drag it to the Trash.
   3. In the Preferences folder, move com.apple.Preview.LSSharedFileList.plist and com.apple.Preview.plist to the Trash.

Once the above is done, launch Preview again, and test if the previous behavioral issues are gone.

This is a volunteer community forum made up of non-Apple employees. Some Apple employees may participate but not in an official capacity. Recently, Apple made it far easier for non-developers to file bug reports with the Feedback Assistant App.

Use the Feedback Assistant App on Catalina or greater to file a bug report.

CMD + Space search on Feedback Assistant and open the App, sign on with your AppleID and submit a report about Preview and it's incorrect behavior. Be as detailed as possible and be sure you can duplicate the problem consistently.

Hey there Aaron Dunsay,

Thanks for reaching out to Apple Support Communities. We understand that you're having some issues saving work in Preview. We're happy to provide some assistance.

Check out this helpful resource:

Manage versions of a Preview document on Mac

As mentioned in that resource, "When you’re actively editing a document, Preview saves a version at least every hour, and more often if you’re making significant changes.".

If you wish to manually save work, look under the "Manually save a Preview document" section.

We hope this helps.

Have a great day!

Thank you for the reply Aaron Dunsay!

Was anything updated or changed when this started occurring? If you manually save as suggested earlier, does the work save correctly?

Also, let's see if this issue occurs in safe mode to help isolate the issue:

Start up your Mac in safe mode

Keep us posted.