If I type pdf in File Explorer search I see an instant list of pdf files in that folder, selecting one in the list opens the pdf file. Typing *.pdf in search searches the folder and produces a complete list in File Explorer of the pdf files in the folder.
Version 1909 build 18363.476

This is expected behavior, by design. If you have problems with search then there is a problem with the install of Windows 10 on your device.
I would suggest system file scans, in the first instance.
Method 1:
Run DISM tool and check the issue. Please follow these steps:
a. Press Windows + X keys and click command prompt (admin). Or PowerShell (Admin).
b. Type the following command in command prompt and press Enter.
DISM.exe /Online /Cleanup-image /Scanhealth
after the scan is complete type the following and press Enter
DISM.exe /Online /Cleanup-image /Restorehealth
then use method 2.
Method 2:
System file checker is a utility in Windows that allows users to scan for corruptions in Windows system files and restore corrupted files.
Run system file checker (SFC) and see if it helps. Please follow these steps:
a. Press Windows + X keys and click command prompt (admin). Or PowerShell (Admin).
b. Type the following command in command prompt and press Enter.
sfc /scannow
After completing the scans restart using Start > Power > Restart.