Can I transfer PDF files from USB to iPad using “lightning to USB 3 camera adapter”?

You don't say what device you need to transfer the files from. USB is only a transfer system using a USB compatible socket.

If the other device has a USB3.0 socket than you should be able to transfer files by that means,

But, there are other means such as Air Drop if both devices have the mutual capability.

There is also data migration from one device to another, neither of these involves using a cable.

The USB3 camera adaptor is only for use between a camera with USB3 capability to transfer videos and photos.

https://www.apple.com/shop/product/MK0W2AM/A/lightning-to-usb-3-camera-adapter