Presumably your iPad is a recent model featuring a USB-C port. With which App are you attempting to copy the PDF files to your thumb drive? Have you previously been able to use this USB drive with your iPad?
Be aware that USB storage devices must be correctly formatted - for Apple devices, typically FAT32, exFAT (FAT64) or APFS with a single data partition. NTFS and other proprietary formats are not supported by iPadOS.
A potential source of the error code is encryption. If the USB drive uses encryption, it will not be accessible to iPadOS unless you have installed an appropriate App. When encrypted USB storage is used with desktop computers (such Windows PC or Mac), either a software driver or a manufacturer-provided companion App must be installed. iPadOS does not support independent installation of driver software, but instead requires an the appropriate App to install the required software to access the drive.