Hi, I'm Robinson, an Independent Advisor and a Windows user like you.
Check if doesn't any corrupt file on the windows.
1-Press Start>Windows System> Command Prompt right-click run as admin and type these commands.
sfc /scannow [Enter]
DISM.exe /Online /Cleanup-image /Restorehealth [Enter]
Then change the default app by file type:
1-Press Start then click on settings
2-Click on apps then click on Default apps
3-Scroll down and click on 'Choose default apps by file type'
4-Search for the .pdf and associate the Nitro pro app