If somehow you are unable to preview .ppt/.pptx/.doc/.xls files in Eagle, the reason being to it could be the following:

The device didn't have Office 2019/ Office 365 installed, or the currently installed version is outdated.
The device may have multiple office versions installed, but the upgrade/uninstall procedure wasn't done properly, leaving a messed-up system Registry.

How to fix

Go to the official site of Office 365
Sign in with your Microsoft Account, then click Download.
In the download popup window, select "other option"
Select "Office - 64 bit"
Click "Install"
Download the installer
Double-click on the installer, the program will run automatically, you'll be able to use after the installation is completed.

Open "Settings > Apps".
Find "Microsoft Office" in the Apps & features list.
Click and select "Modify".
Select "Quick Repair", then click "Repair".
Click "Repair".
The repair progress will be finished within approx. 3 minutes.
Right-click the file in question, and click "More" > "Refresh Thumbnail" to see if the issue is fixed.

Click the Start menu to the bottom left, type to search "regedit".
Click to run Windows Registry Editor.
Enter "Computer\HKEY_CLASSES_ROOT\TypeLib\{00020813-0000-0000-C000-000000000046}" in the path, then press enter.
Find folders named as "1.8" and "1.7" (keep "1.9" or higher folders) in directory "00020813-0000-0000-C000-000000000046", right-click and delete them.
Right-click the file in question, and click "More" > "Refresh Thumbnail" to see if the issue is fixed.

Note: Whether or not the Office is activated, Eagle can import .doc, .ppt files or so as long as the Office is installed properly

Note2: Office files encrypted by third-party tools may not be imported in Eagle.

Unable to preview .ppt/.doc/.xls files