I can't find pdf files sent via airdrop on my ipad

Addressing your specific problem, to find your lost PDF files, try using your iPad’s built-in Search function:

Use Search on your iPhone, iPad, or iPod touch - Apple Support

If you know the name of the file - and the file was actually saved from the AirDrop transfer - it should be located. The native Search is the only search-tool that can search across all App containers.

Those PDF files may not have gone anywhere because you did not specify where and what app you wish to save these files to on yout iPad

Have you checked the Files app under Downloads to see if any of those PDF files were downloaded to that location?

If not, no files were downloaded/transferred to your iPad.

You need an app,,on your iPad capable of storing and openmg PDFs,

That app may need to be a notes app ( possible Apple's own free Notes app ) or a third party notes app, a third party text or document app or a dedicated third party PDF reader or reader/editor app ( Goodnotes should have been fine, but if not, specifically told to save the PDFs there, then nothing would AirDrop/be sent/transferred there.

Just because Apple AirDrop states a file was sent, doesn't, neccessarily, mean the receiving device received anything.

Best of Luck to You!

Tap and review these Apple support links below.

How to use AirDrop on your iPhone or iPad - Apple Support

Use AirDrop on your Mac - Apple Support