Can you tell us what happens every time you print a or clicked the Print Preview button? There are times that a cache or browser issues can cause the print function to not work properly.
Let me give you some steps that can resolve common printing issues in QuickBooks Online.
Start by switching to an incognito or private browsing session. These keyboard shortcuts can start a session depending on the browser that you're using:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox
Control + Option +P if you're using Safari
Once done, log back in to your account and check if you can already see the preview or print the transaction.
Sometimes, changing to a different browser can also resolve PDF viewing issues. You can use any of the mentioned browsers above.
There's also a possibility that the Adobe Reader itself is damaged. Follow the troubleshooting steps provided in these articles in case you're still getting the same issue: